Details are provided by Ross McKitrick in a background paper: The Nunes Memo and the Horowitz Report: Popcorn  Edition1 Ross McKitrick  January 29, 2018

I find the backstory to be as gripping as any paperback spy thriller. But the plot is
complicated and there are many moving parts. Herein, based on my readings, is my surmise
about what the 4 page Nunes memo contains, and also what a forthcoming report by
Department of Justice Inspector General Michael Horowitz will conclude. I have included
some source citations but left many out since there are too many to list.

Chapter 1: Benghazi and Hillary’s Emails

Chapter 2: The Clinton Email Investigation

Chapter 3: Steele Visits the FBI

Chapter 4: The DoJ Inspector General Investigation

Chapter 5: The Trump Tower Wiretap

Chapter 6: Mueller Appointed Special Counsel

Final Chapter: The Two Reports

The Onion weighs in:  

FBI Warns Republican Memo Could Undermine Faith In Massive, Unaccountable Government Secret Agencies

WASHINGTON—Stressing that such an action would be highly reckless, FBI Director Christopher Wray warned Thursday that releasing the “Nunes Memo” could potentially undermine faith in the massive, unaccountable government secret agencies of the United States. “Making this memo public will almost certainly impede our ability to conduct clandestine activities operating outside any legal or judicial system on an international scale,” said Wray, noting that it was essential that mutual trust exist between the American people and the vast, mysterious cabal given free rein to use any tactics necessary to conduct surveillance on U.S. citizens or subvert religious and political groups. “If we take away the people’s faith in this shadowy monolith exempt from any consequences, all that’s left is an extensive network of rogue, unelected intelligence officers carrying out extrajudicial missions for a variety of subjective, and occasionally personal, reasons.” At press time, Wray confirmed the massive, unaccountable government secret agencies were unaware of any wrongdoing for violating constitutional rights.