First, you need to dispense with the "firsthand" requirement.  This apparently is not only easy for the I.G. to do, but easy to do in secret.  Coincidence?  As Sean Davis points out:

Between May 2018 and August 2019, the intelligence community secretly eliminated a requirement that whistleblowers provide direct, first-hand knowledge of alleged wrongdoings. This raises questions about the intelligence community's behavior regarding the August submission of a whistleblower complaint against President Donald Trump. The new complaint document no longer requires potential whistleblowers who wish to have their concerns expedited to Congress to have direct, first-hand knowledge of the alleged wrongdoing that they are reporting.

Once it's no longer about what someone personally experienced, it can then be an emetic farrago of rumor and gossip.  This is the chance to throw everything in.  All the leaker doesn't like or disagrees with or pretends he "heard it from a friend who heard it from a friend who heard it from a friend" can be included.  Without the firsthand requirement, it can be about really anything that will grab the media and the masses. 

It can also be formulated as a dossier, with references to media stories, and since it's not direct, it can say early and often things, as Sean Davis points out:

The document itself is riddled not with evidence directly viewed by the complainant, but repeated references to what anonymous officials allegedly told the complainant: "I have received information from multiple U.S. Government officials," "officials have informed me," "officials with direct knowledge of the call informed me," "the White House officials who told me this information," "I was told by White House officials," "the officials I spoke with," "I was told that a State Department official," "I learned from multiple U.S. officials," "One White House official described this act…" et seq.

And footnotes, don't forget footnotes — they make a dossier look professional.  It doesn't matter what they say; nobody reads them.  And articles, cite and quote lots of articles; it makes the writer look intelligent and well read.  Then you simply must get the right audience; it's kind of like "judge-shopping," except you are "I.G.-shopping."  Once you have the right man, as Julie Kelly points out:

Among the officials working with CrowdStrike on the exercise was John Carlin, Mueller's former chief of staff and then head of the Justice Department's national security division. Carlin facilitated the October 2016 FISA warrant for Trump campaign aide Carter Page; he also received regular briefings from then-deputy FBI director Andrew McCabe on both the Clinton email investigation and the Trump-Russia collusion probe. (Michael Atkinson, the intelligence community's inspector general at the center of the so-called whistleblower controversy, worked directly for Carlin in 2016 and 2017.)

And lie, you have to lie; it sets the stage for all the Democratic lies to come.  Take Adam Schiff, who opened his inquisition...I mean hearing with a two-minute hate speech, including everything in his "parody" that every lefty wished Trump had said.  He knew that that moment was when viewership would be highest and his words would probably be the only thing many would remember, especially those who want to believe. 

We've been very good to your country, very good. No other country has done as much as we have. But you know what? I don't see much reciprocity here. I hear what you want, I have a favor I want from you, though. And I'm going to say this only seven times, so you better listen good.

I want you to make up dirt on my political opponent, understand, lots of it, on this and on that. I'm going to put you in touch with people, not just any people, I'm going to put you in touch with the attorney general of the United States, my attorney general Bill Barr. He's got the whole weight of the American law enforcement behind him.

Trump never said any of that, but lies make for compelling TV.  Repeat them often enough, and people will remember them.

A month ago, Adam Schiff tweeted, "Trump is withholding vital military aid to Ukraine, while his personal lawyer seeks help from the Ukraine government to investigate his political opponent.  It doesn't take a stable genius to see the magnitude of this conflict.  Or how destructive it is to our national security,"

Don't be so jaded; it's just another coincidence.

Once you've done all that, you can count on the skirling choristers in the media to do their job.  That would be keeping it in the public eye and portraying that 30-minute conversation as the most corrupt thing to have ever been done by a president.

How dare he?

Please follow William L. Gensert on Twitter at @williamlgensert.