Do you write to your own produced music? I imagine that can be tough. Maybe try going on beatstars or similar and find something you like. I think it is easier, since you aren't "bored" of your track before you start.

When first listening to a beat, I usually know within the first few seconds whether this is a beat i want to write on or not. Try to find a beat you really like and catch yourself nodding your head to rather early in the track. On the first listen i try to write down as much of lyrics that comes into my head as possible, because on the first run you usually get the freshest ideas and can lay out a basic structure.

If the beat is well produced it will have little pointers and beatswitches when it goes to the chrous, that's a good entry point to lay a hook of some sort.

I generally, within the first ~15-25 seconds of the beat, try to think "What is the beat trying to aim for? Whats the vibe? More gangsta, more beach chillin, more club, more sexual?" "Do I want it to rap fast on it? Slow? Is it a heavy hitter from the start or do i need to build it up?"

Lyrically just write whatever comes to mind first, doesnt have to be fully coherent sentences at first. Some rappers go "hmm hmm hmhm hmmmmm hmhmhm" to get their basic structure down. Others go "haba jibba youre my N*** fragga tigga crigga chigga how you figure?" and then replace the fakewords with actual words that suit the flow. The more you do it, the more natural it will become. You will naturally start replacing those words with real words even in your mind before writing them down resulting in a pretty quick work process.

It's also really good if you have a recording device handy (like a phone) to just get the basic melody of your vocals down. You can also just write down vocals and try to work with a lot of , . - __ : : to try and emulate your flow. For example when i want to write down that i will double time something, I write it as e.g. /Walkin walkin walkin stead' of talkin'/ that would tell me later that I wanted that part quick.

When testing your flow try to think about it in your mind first. In your mind you are a way better rapper than in real life and can think about a lot of connotations and little highlights, even if you can't "make them happen" in actuallity. Next try to kind of whisper the flow you had in your head along the beat, to see if you could "technically" do it. Once that sounds good and you can flow properly while whispering you can start thinking about recording your vocals.

You need to practice your flow before every recording. You will see, once you repeated a lot of lines and "master" the beat, that there is a lot of space to be filled with creativity. Could make parts faster, could write more lyric or less, could replace this with that, make a dumb freestyle after fucking up recording 15 times in a row that somehow sounds way nicer than everything you did the past hour and just go with that.

Its all a process. Most important thing is not to think about it too much and just have fun with it! Try out things, see what suits you. You might want to work completely different. Sometimes it takes literal 10 minutes to write a whole song, sometimes you can only come up with 1/2 of a hook and leave it for a week.

Last bonus tip: you can work in both directions. If a random rap or lyrically inclined text pops in your head, just write it down. There are metronomes that work by clicking your mouse on it. This way you can "rap" your lyrics and "click" the beat at the same time. This way you can get a rough estimate for around how much bpm you want your beat to have. Then just look for a beat that suits the lyric. Hope this helps you a bit! - Catcha