Alright. Flowing in rap music is difficult. It is not easy, but it is doable. Flow basically means what we call being in the pocket. And the pocket usually means that you're not interfering with any other instrument and you're falling on the beat in a way that feels simple and as you're one. Almost as if the beat is like a horse, and you're just riding the horse. You're just in the wave.
One key way to know that you're in the beat is, is if you can bob your head to the beat, and there's no kind of interference where you're going off the beat. So, knowing that you're on the beat that when you're rapping you just fall in right on the beat. You'll notice that when you go to a good rap show, or you're listening to a good rap song, your head will just kind of just flow to the same way that the rapper is rapping, because that person is on the beat and riding the beat.
To acquire this skill to ride the beat really takes just a lot of practice, and taking certain words out. A lot of times, we put too many words in the rap lyric, and that's why it's not flowing in the way it should.
One way to ride the beat is to say less. Sometimes, rappers fall off the beat because they put too many words in. Unlike writing correct English, you don't have to put every detail into the rap song. What you want to do is take out simple words like, "I", "and", "but". You don't have to say all those things. In a sense people kind of fill in the blanks, and it also leaves you space to take a breath and flow on the beat in a better way, because you're not trying to say as much and you can glide on the beat.
More often than not, flow really just takes practice. Doing it over, and over, and over again. Whenever you feel like, "Alright. Well, maybe I'm really not on the beat," you take out a word or two and it will flow like butter. That's how you learn the flow on a beat.