Choose a rapper for whom to write. Ideally, you should pick a performer or aspiring rapper you know personally who will later be able to learn the verses directly from you. Ask if the rapper would like to try out your lyrics; if you are turned down, don't take it personally, as many rappers pride themselves on their own writing. Ask around.
Study your performer's vocal style. If possible, have the rapper recite a verse out loud. Pay attention to cadence, breath control and any accent or particular words with which the rapper has trouble. You will need to consider these strengths and limitations when writing.
Find a rap beat or have your performer give you one. Write the rap song one line at a time, thinking as you go about what inspires your performer and what topics are believable for the performer to rap about. Rehearse what you have often, and try to hear your performer's voice rapping your words.
Write and continue to edit line by line until you have three to five minutes of music. Make a copy of your finished lyrics and schedule a time to teach them to your performer.
Rehearse your ghost lyrics with your rapper, demonstrating the rhythms and emphases of each syllable. Encourage feedback and be open to further revisions if your performer struggles with certain lines. When you are both satisfied, enjoy listening as a rapper performs or records a rap song written by you.
Study different subgenres of rap, such as gangster rap, lyrical hip-hop, political rap and different regional adaptations. Practice ghostwriting for various kinds of rap songs and collect writing samples as well as recordings of your past work. Ask around at local hip-hop venues and e-mail record labels to find bigger names to write for as you climb the ranks of rap-song ghostwriters.