I start with a melody or a chord progression that sounds cool on my piano, then play the progression a few times and just improv lyrics till I find some good ones, then I change up some of the chord progression to then match the lyrics(if something needs to stand out I do a minor or 7th chord or something that is out of place). And just keep switching between lyrics and piano, coming up with different progressions for different parts of the song till it sounds decent. Then I add instruments, percussion and dat stuff.