Write on paper or a blank word processing document one line of your rap song. This will equal one bar. An average verse in a rap song will have 16 bars.
Write on the empty line directly under your first bar a second line of the song. This will be your second bar. Bar two does not have to be the same length as the first, although it should be close. End your second bar with a word that rhymes with the last word of your first bar. For example if your first bar ends with the word "mix," your second bar should end with a similar-sounding word like "fix." Writing more complex rhyme schemes throughout each line will create a better-sounding bar.
Skip one space on your page. Repeat steps 1 and 2. Continue with the same ending rhyme throughout bars four and five. Skipping a space will allow room for you to edit the bars you have already written. Although this is not required, it will help keep your verses legible.
Repeat steps 1 through 3 until you have 16 bars. This will make one verse for your rap song. Alternate rhyming words at the end of your bars every two to four bars. This will add complexity to your rhymes.
Sing or "rap" your bars over your music. End each bar at the end of every four count on your music. This can be done by counting your snare drums on a basic drum beat. For example, "kick, snare, kick, snare," will equal a four count (kick being your bass drum). End your bars' rhyming words on the last snare of each four count. Doing this will ensure you stay with the rhythm.
Connect all 16 bars together without any breaks in between on your music to complete your first verse. Repeat as necessary depending on how many verses you have in your song. When rapping your bars over the music take time for each bar to "engineer" how it will fit on the beat.