So, when you write a rap chorus you want to start off by making it as simple as possible. So, if your song is "I'm the baddest", just say that I'm the baddest, I'm the baddest, I'm the baddest right? by starting off simple it will give you a direction for actually writing the rest of the song and through that creativity of writing the verses, you'll come up with maybe something more creative on the chorus. Primarily, the simplest way, and this is why so many people love hip-hop is get to the point. Just say what it is. If "I'm the baddest in Brooklyn, I'm the baddest in L.A, I'm the baddest in the world there's nothing you can say," whatever.
You start out saying exactly what you want to say and then, from there, you can evolve it. Many people block themselves in writing their choruses because they're trying to figure out the way it should final be and that means that they don't always have focus on the process. The process is to start off by just saying exactly what it is and then add on as you finish writing the song.
Another way of writing a rap chorus is by taking a popular song, a popular old school song, and just using their chorus. You will see this time and time again in many remakes, where people take a popular song, and just use that same chorus, and add a little twist.
Somebody who's used this a lot is Puff Daddy. If you study a lot of Puff Daddy's songs , and his collaborations with Mase or Biggie Smalls, he used a lot of old school disco songs that were popular in the '80's and then he added his own twist to those songs. One way of writing a course is using a popular song that has already been created.
If you're either using a popular song that has been created, or taking exactly what you want to say, what is the thing you want to drive home to the audience, and just repeating that over and over, and then allowing time to evolve as you write the rest of the song and tweaking the chorus as you get closer.
But, for the most part, what people most love about hip-hop is about getting to the point. So, say exactly what you want the audience to leave with, and that is how you make your chorus.