I know that there is the FriendlyBaron speedrun guide but I want to know if there is another or should i watch that or watch speedrun runs and practice chapter by chapter

I'm pretty new to GTAV speedrunning, just recently did my first trevor% run (more of a test run than anything) I watch a lot of DarkViperAU so i learned a lot of strats from him. Streams or highlights, whatever suits you best. Apply those to a run, remember what parts you have difficulties with and study them more extensively. Hope this help you a bit!

Burhac recently did a Trevor% tutorial. For the other segments, there is FriendlyBaron's Casual vs Speedrun series and his GDQ submission run (where he did a full run talking about all the strats), but unfortunately they are somewhat outdated now. So you should also watch the current top runs to catch all the new optimizations.