Write your lyrics. Rap is considered by many as a form of poetry and should contain all the fundamentals of good poetry. Make certain that you have originality, metaphor, simile, creative expression, alliteration and a emotional conveyance.
Choose your style of presentation. Some styles are monotone, which allows an emphasis on the lyrics. You can also use a machine gun style that aggressively emphasizes specific words. Find an original style of delivery that is best for you and your music.
Arrive at the flow of the music. Find the rhythm and timing for the song. There will be a different cadence for a song about a break up than one that is about falling in love.
Build your own image up in the song. Rapping words flatter the rapper by telling the world that they are great. Make certain that you include how great you are and how unlucky the other person was to break up with you.
Use internal rhyme in your rap song. Besides rhyming lines you need to rhyme the insides of the lines. "I'm a kind blind find who my woman let go" is an example of internal rhyme.
Add in rhythmic noises that sound like percussion. All rap seems to have a vocal band somewhere in the song.
Insult the person that broke up with you. Sorrow seldom is the mode of rap; anger takes priority.