Brainstorm the subject matter on which you wish to rap on. Write down all words and phrases that relate to the subject of your choice. If the subject is basketball, write down all ideas in relation to basketball such as "slam dunk," "team players."
Write the first line of the rap and think of another line to rhyme with it. The last word in each line should rhyme with each other. For example, "I just can't pass the game up/ I'm looking for a slam dunk." Use your subject to determine the next line in your rap.
Write another line and ensure that you use a metaphor for the second line. Use the words "like" or "as" to make comparisons between your first line and second line. Compose 16 punchlines, each in sets of two, and recite the entire rap.
Repeat the above steps to compose more punchlines.