When you write lyrics for a rap song, either tell a story or write about ideas or concepts. Choose your lyrics and form them into a poem or free verse. Think about laying it over a back beat. Here are some catchy and playful rap lyrics from A Tribe Called Quest in "Jazz (We've Got)":
"Stern firm and young with a laid-back tongue/ The aim is to succeed and achieve at 21/ Just like Ringling Brothers, I'll daze and astound/ Captivate the mass, cause the prose is profound."
A good vocal hook enhances your lyrics like the icing on the cake. Often the chorus carries the vocal hook first heard after the first verse of lyrics. Sing your vocal hook. Write it two to four lines long. Here is a vocal hook by Alicia Keys from her and Jay-Z's collaboration "Empire State of Mind": "Concrete jungle where dreams are made of / There's nothing you can't do / Now you're in New York /
These streets will make you feel brand new."
Make your beats easy to dance to with a solid back beat emphasizing beats two and four. Rap songs tend to be in 4/4 time so the emphasis is on the beats two and four in a four-beat measure. Sound effects and samples can come from car horns, vinyl scratching or even vocal samples with effects layered on them. Recording programs such as Garageband or ProTools have these effects in their sample libraries.