With extreme difficulty.
The only real step 1 — unless you want to get into a philosophical argument about the foundation of knowledge and the Münchhausen trilemma — is "get a law degree", which for most people is much too hard.
The closest that nerds like me can do is to watch another lawyer explain what Trump's lawyers did wrong, and hope that YouTube isn't just one giant conspiracy by The Man (and while I have reasons for trusting this channel specifically, I do not trust YouTube collectively): https://www.youtube.com/watch?v=Lhy5Y8xVHS0
(This is, I think, further from a law degree than "watching a 25 minute video about Excel" is from "senior software engineer").