Figure out what you actually want to say in your rap. Lots of rhyming does you no good if your rap is complete nonsense.
Pick a phrase that is crucial to your rap that you wish to rhyme with. For example, if you are doing a rap about hunting, you may want to rhyme with the phrase "hit the bear"
Make a list of words that rhyme with both of your key words. Unlike a regular rhyme where you'd only rhyme with "bear" (hair, there, care, chair), in a multi you'll also want to rhyme with "hit" (sit, get, spit, quit).
String together pairs of words from your two lists that make sense together. For example, you may have sit and chair, get and there, quit and there.
Flesh out your word pairs into fuller phrases. For example "I was in my SITting CHAIR", "needed to GET to THERE", and "had to QUIT from THERE."
Figure out how to work your phrases into a logical structure that involves your key phrase. For example, you were sitting in your chair, went to get somewhere to go hunting, shot at the bear but missed, and had to run away.
Arrange your phrases in the order that makes sense for the story, and add lyrics to make it a full verse. For example:"Fed up with being in my sittin' chair,Want to go hunting, had to get to there,Took a few shots, but didn't hit the bear,And the grizzly got pissed, so I quit from there."