SWIM's recommendation for anyone wanting to learn is to sign up for Breachforum and read up on all of the contents in their OSINT subforum [1] where you can ask questions too. If you're willing to boogey on into the grey area, you can even download leaked datasets from BreachForum, Leakbase.io, etc. to make your job a lot easier if it involves identifying people online - for example a few databases like AT&T customers and the NationalPublicData database were leaked recently which is kind of like having a private LexisNexis Accurint at your fingertips.
This is definitely a topic area where people overly familiar might omit standard safety precautions within their instructions: Do use a burner VM, don't do any of your normal computing on that VM, etc.
[1] https://breachforums.st/Forum-OSINT