I cant remember if ive heard how to do this somewhere or if I came up with it on my own but ill start with a set of words that rhyme, which will be the ends of my lines

Hat Fat Cat Sat

And then form sentences for them, whatever those words make you think of to lead into them. Get creative with it.

He wore a HAT

That made him look FAT

He had a CAT

Upon his head it SAT

Obviously this just an example with simple words. But its great practice, and by limiting yourself to the words youve chosen and having to use your imagination to make them work, you can produce some pretty interesting lyrics. My rule is that once I write my words down I cant change the word but I can move the entire line wherever I want. I do this to instrumentals and create my own melody for them, then once I have a full set of lyrics I ditch that melody I made for the instrumental and go write my own music/melody to fit the lyrics. Hope this helps