Everyone writes lyrics differently, it seems. For me it varies, but a typical song could go something like coming up with a guitar part or chord progression I'm feeling and at times I might just start singing random stuff over it and something might stick. A lot of the times the vibe of the guitar parts I come up with will set a mood and that'll put a theme in my head. Then I'll start trying to build what I want to say off of that. The tricky part is finding a way to say what you want to say, in a way that flows properly. There's also the fact that depending on the vocal melody, certain words/letters/etc. will either sounds good or bad. What I mean is for example, in a soft song I might not want harsh sounding words because they won't flow properly for that song/vocal melody. There have been times where a word didn't sound good with a vocal melody so I had to find another word to say something similar, because I wanted to keep the vocal melody as it was.
Some people say they write lyrics first. I don't usually do that. I do sometimes write one liners I might come up with that I think would be cool to pull from in future songs. What some people do when they write lyrics first is write in some way that flows properly, like in poem format for example. Then they'll either try to write music to that, or if they have something not written specifically for those lyrics, they may try to rework the lyrics to fit the guitar parts already written.
The best way to practice is either to just practice writing poems about things you're feeling or want to talk about, or stories you want to tell. Or jam out on guitar, piano, or whatever other instrument and try to let the lyrics flow from you based on how you're feeling and vibing from whatever you're jamming out to.
No idea if this was helpful or just really confusing haha Hope it helped somewhat xD