Come up with a structure for your rap. Raps usually follow a 4/4 beat and use around 13 or 14 syllables in every line.
Brainstorm what you want to write. You can write your MC lyrics about your own skills, where you're from, a friend or relative, a feud or anything else you're interested in.
Start writing your lyrics using end rhymes. Ideally, each line should end with a rhyme, and the lyrics should tell a story.
Add in some multis to make your rap more sophisticated. In multis, two or more words at the end of a line rhyme with the same number at the end of the next line. For example, if one line ends with the words "bake a cake," the next might end with "break a rake."
Add in-rhymes into your rap. In-rhymes are rhymes or near rhymes between words in the middle of a line instead of at the end. For example, in the lines "I'm LIKING grandma's ICING when she bakes a cake/but she's HATING and BERATING when I break a rake," the bold words LIKING and ICING form an in-rhyme, as do HATING and BERATING.
Let the rap sit for a week when you finish it, then come back and read it out loud. Write down any changes that occur to you and rewrite your rhymes.