Glenn Simpson and his firm, Fusion GPS were hired to do opposition research on Trump, first by a conservative donor and later by Hillary Clinton's campaign.

GOP mega-donor and hedge fund billionaire Paul Singer, who funds the conservative website The Washington Free Beacon, hired Fusion GPS' in the Fall of 2015 to dig up dirt on then-candidate Donald Trump. The project was funded by the conservative from fall 2015 to spring 2016, when it pulled its funding as Trump looked set to clinch the nomination. (Source: Breitbart 10/27/17)

While supporting Republican establishment favorites such as Rubio and 2012 presidential candidate Mitt Romney, Paul Singer was a major backer of Common Core and was the founder of a super-PAC that has the express purpose of turning the GOP pro-gay marriage. (Source: Breitbart 10/27/17) The firm was hired to dig up information about several Republican presidential hopefuls, including Trump whom Singer particularly disliked.

Fusion GPS was next hired by Hillary Clinton and the Democratic National Committee in mid-2016. Fusion GPS would build on a foundation of research already compiled, and a lengthy, well documented trail leading to Russia already existed.

Trump has been hungry for Russia projects for more than three decades. He has repeatedly touted plans for a Moscow mega-development and has courted a steady stream of investors from the former Soviet Union for ventures in New York, South Florida and other locations. Trump bragged to a New York real-estate publication after a November 2013 dinner with prominent business leaders: “The Russian market is attracted to me.” Trump’s business interest in Russia began in 1986. He met Soviet Ambassador Yuri Dubinin at a luncheon and, as he recounted in his 1987 book, “The Art of the Deal,” the two began “talking about building a large luxury hotel across the street from the Kremlin.” Trump flew to Moscow in July 1987 with his first wife, Ivana, to check out potential sites. Trump wrote in his book: “It was an extraordinary experience.We stayed in Lenin’s suite at the National Hotel, and I was impressed with the ambition of the Soviet officials to make a deal.” The hotel deal stalled.

Trump explored the Russian market again in 1996, with help from his friend Howard Lorber, who is chief executive of Vector Group, a holding company that back then owned a Russian cigarette company and now owns Douglas Elliman Realty, one of the leading brokerage firms for super-rich Russians seeking property in the United States.

Trump announced plans for a $250 million investment that would include a “Trump International” complex at a November 1996 news conference in Moscow. “We have an understanding we will be doing it,” he said.

Trump bragged about his plans in January 1997, when he and Lorber met visiting Russian politician Aleksandr Lebed in New York. A 1997 New Yorker profile of Trump captured their exchange and showed the breadth of Trump’s hopes for Moscow investment and business connections.

A Reuters investigation found that 63 people with Russian addresses or passports have purchased $98.4 million of property in Trump-branded condos in South Florida. Trump also had a personal infusion of Russian cash in the liquidity-starved 2008 market. That year he sold for a handsome $95 million a Palm Beach waterfront mansion he bought at auction in 2004 for just $41.35 million — more than doubling his money at a time when much of the South Florida market was underwater. The buyer was Russian oligarch Dmitry Rybolovlev.

“In terms of high-end product influx into the U.S., Russians make up a pretty disproportionate cross-section of a lot of our assets. We see a lot of money pouring in from Russia,” said Donald Trump Jr. according to a Sept. 15, 2008 article. He said he had made a half-dozen trips to Russia during the previous 18 months.

The apex of Trump’s personal fascination with Russia may have been 2013, when he brought the Miss Universe pageant to Moscow and talked, yet again, of building a “Trump Tower” there. As Trump began hyping the pageant, he even tried to draw in Russia’s president himself, tweeting on June 18, 2013: “Do you think Putin will be going to The Miss Universe Pageant in November in Moscow — if so, will he become my new best friend?”

Suffice it to say that there exists a lengthy and well documented record of a Trump / Russia relationship which continues to the present day.
(Source:Washington Post Nov.2, 2017 "A history of Donald Trump’s business dealings in Russia")

The new client of Glenn Simpson's Fusion GPS (HRC/DNC) had given his firm an investigatory assignment with wide latitude

Simpson picked up the phone and called England.He knew exactly who he needed on this assignment. There is a row of Victorian terraced houses on a side street in London’s Belgravia district, each projecting a dowdy respectability with its stone front steps leading to a pair of alabaster pillars and then a glossy black door. A small, rectangular brass plate adjacent to a glossy black door on an older Victorian. Its dark letters discreetly announce: ORBIS BUSINESS INTELLIGENCE, LTD.

In their previous lives, the founding partners of Orbis, trained and nurtured by the Secret Intelligence Service, had been in the shadowy business of finding out secrets in the name of national interest. Now they performed more or less the same mission, only they had transferred their allegiance to the self-interests of the well-paying customers who hired them. And so it was that in June of 2016, Christopher Steele, ex-Cambridge Union president, ex-M.I.6 Moscow field agent, ex-head of M.I.6’s Russia desk, ex-adviser to British Special Forces on capture-or-kill ops in Afghanistan, and a 52-year-old father with four children, a new wife, three cats, and a sprawling brick-and-wood suburban palace in Surrey, received in his second-floor office at Orbis a transatlantic call from an old client.

The investigation mandate from Fusion GPS was for Steele to carry out an “open-ended” examination of Trump’s business record, including his bankruptcies.

From 2004 to 2009 Steele had headed M.I.6’s Russia Station, the London deskman directing Her Majesty’s covert penetration of Putin’s resurgent motherland.
He could count on an army of sources whose loyalty and information he had bought and paid for over the years. There was no safe way he could return to Russia to do the actual digging; the vengeful F.S.B. would be watching him closely. But he had active contacts. A lot of them.

"The investigation evolved somewhat quickly into issues of Trump's relationship to organized-crime figures.”

How good were Steele's sources? Consider what Steele would write in the memos he filed with Simpson:

Source A—to use the careful nomenclature of his dossier—was “a senior Russian Foreign Ministry figure.”
Source B was “a former top level intelligence officer still active in the Kremlin.” And both of these insiders, after “speaking to a trusted compatriot,” would claim that the Kremlin had spent years getting its hooks into Donald Trump.
Source C was not identified.
Source D, “a close associate of Trump who had organized and managed his recent trips to Moscow.”
Source E was “an ethnic Russian” and “close associate of Republican US presidential candidate Donald Trump.”
Source F, “a female staffer” at the Moscow Ritz-Carlton hotel, who was co-opted into the network by an Orbis “ethnic Russian operative” working hand in hand with the loquacious Trump insider, Source E.
Two sources told quite a lurid story, the now infamous “golden showers” allegation, which, according to the dossier, was corroborated by others in his alphabet list of assets. It was typical for Moscow Center to have Trump's suite wired up for sound and video (the hotel’s Web site, with unintentional irony, boasts of its “cutting edge technological amenities”), Steele apparently began to suspect that locked in a Kremlin safe was a hell of a video, as well as photographs.

(Source: Vanity Fair March 2017 - How Ex-Spy Christopher Steele Compiled His Explosive Trump-Russsia Dossier)

Steele reported his concerns to Fusion GPS that the incoming US President appeared to have been compromised by Russia and could be subject to blackmail.
They agreed that a discrete warning had to be issued.

In early October, on a trip to New York, Steele sat down with David Corn, the 58-year-old Washington-bureau chief of Mother Jones. If he agreed to protect a source, his commitment was unshakable. Steele’s identity would be safe with him. Corn accepted the terms, listened, and then went to work. He began to investigate, trying to get a handle on Steele’s credibility from people in the intelligence community. And all the while the clock was ticking: the election was just a month away. On October 31, in what one of Corn’s colleagues would describe as “a Hail Mary pass,” he broke a judicious, expurgated version of the story—“A Veteran Spy Has Given the FBI Information Alleging a Russian Operation to Cultivate Donald Trump.”

In early November, Corn shared a bit of what he knew with Julian Borger, of The Guardian. And Simpson, during a sandwich lunch with Paul Wood in the BBC’s Washington radio studio, reached into his briefcase and handed over to the British journalist a redacted version of Steele’s initial report. It wasn’t long before, as The New York Times would write, the memos by the former spy “became one of Washington’s worst-kept secrets, as reporters . . . scrambled to confirm or disprove them.”

Then, on November 8, Donald Trump was elected president of the United States.

Within hours of the president-elect’s victory speech, Vladimir Putin went on Russian state television to offer his congratulations. And the Popular Front, a political movement founded by the Russian president, slyly tweeted, “They say that Putin once again beat all.”

In late November in Nova Scotia, John McCain was one of 300 attendees at the annual Halifax International Security Forum. At some point that busy weekend, Senator John McCain and David J. Kramer, a former State Department official whose bailiwick was Russia huddled with Sir Andrew Wood, a former British ambassador to Russia. Sir Andrew, 77, had served in Moscow for five years starting in 1995, a no-holds-barred time when Putin was aggressively consolidating power.Sir Andrew knew Steele well and liked what he knew. All three had heard the rumors about Steele’s memo. Sir Andrew told The Independent newspaper; “We spoke about how Mr. Trump may find himself in a position where there could be an attempt to blackmail him with kompromat.” A decision was reached. On an evening about a week later, using a ticket purchased with miles from his own account, Kramer flew out of Washington and landed early the next morning at Heathrow. Veteran intelligence protocol was observed to make certain each party was in fact who he said he was. The two talked for hours and Steele passed what intelligence analysts believe was a copy of the full, expertly crafted and fully sourced report to Kramer. On December 9, McCain sat in the office of F.B.I. director James Comey and, with no other aides present, handed him the typed pages. In the waning days of the Obama administration, both the president and congressional leaders were briefed on the contents of the Steele memos. And in early January, at the end of an intelligence briefing at Trump Tower on Russia’s interference in the presidential election conducted by the nation’s top four intelligence officials, president-elect Trump was presented with a two-page summary of Steele’s allegations.

BuzzFeed, full of journalistic justifications, posted the entire 35-page report online. Then The Wall Street Journal outed Christopher Steele as the former British intelligence officer who had authored the Trump dossier. And next Steele, who in his previous life had directed the service’s inquiry into the death of Alexander Litvinenko, the former F.S.B. officer who was fatally poisoned by a dose of radioactive polonium-210, quickly gathered up his family, asked a neighbor to look after his three cats, and headed off as fast as he could to parts unknown—only to return nearly two months later to his office, refusing to say little more than that he was “pleased to be back.” His arrival was, in its guarded way, as mysterious as his disappearance.

A grim case can be made that the Russians are taking the memos seriously. Oleg Erovinkin—a former F.S.B. general and a key aide to Igor Sechin, a former deputy prime minister who now heads Rosneft, the giant Russian oil company, and whose name is scattered with incriminating innuendo through several memos—was found dead in his car the day after Christmas. The F.S.B., according to Russian press reports, “launched a large-scale investigation,” but no official cause of death has been announced.

The F.S.B. arrested two officers in the agency’s cyber-wing and one computer security expert, charging them with treason.

Three members of the Trump election team were mentioned in the dossier for their alleged ties to Russian officials—Paul Manafort, the former campaign chairman; Carter Page, an early foreign-policy adviser; and Roger Stone, a longtime ad hoc adviser.

According to The Washington Post, the F.B.I. in the weeks before the election grew so interested in the contents of the dossier that the bureau entered into a series of conversations with Steele to discuss hiring him to continue his research. Once the report became public, however, the discussions ended, and Steele was never compensated.

But ultimately, in any examination of the veracity of an intelligence report, professionals weigh the messenger as heavily as the news. Steele’s credentials were the real thing and, apparently, impressive enough to scare the hell out of James Clapper, the director of national intelligence, James Comey, John Brennan, the C.I.A. director, and Admiral Mike Rogers, the N.S.A. director. How else can one explain their collective decision to pass on the still-unverified dossier to the president and the president-elect?

Finally, but not least, there is Steele’s own tacit but still eloquent testimony. Retired spies don’t go to ground, taking their families with them, unless they have a damned good reason.

(Source: Vanity Fair March 2017 - How Ex-Spy Christopher Steele Compiled His Explosive Trump-Russsia Dossier)