Select a few of your favorite rap artists and listen to your favorite songs. Download or print the lyrics to those songs. Examine them, underlying the words that rhyme. Listen to the songs again, rapping along with them and emphasizing the rhymed words at the end of each line.
Write a short, simple rap song on a piece of paper or on a computer. Use a thesaurus and dictionary as you write. Make sure the words at the end of each line of your rap sound alike. Break up lines naturally into thoughts or statements. End them when you need to pause in your rap.
Decide upon a "filler" phrase that you can insert as needed while rapping live in a freestyle manner. Use this filler to prevent awkward pauses while you're thinking of the next sentence of your rap.
Create rap songs about yourself or a subject about which you are passionate so you can express yourself and have plenty of material. Include metaphors and reference current events in your rap songs.
Practice delivering your rap song by yourself or with a few friends to get ideas about word changes and other raps you can write. Hold freestyle practice sessions with a few friends to develop your style and flow. Develop good flow by continually expanding your vocabulary, practicing freestyle rapping with friends and listening to hip hop music by artists you enjoy.