Start with a simple tongue twister like "She sells seashells..." or "How much wood could a woodchuck chuck..." These are important because they emphasize making distinctions between words which sound similar. Spend at least 20 minutes a day practicing these in the beginning.
Select a rap song with a moderately fast rap, and learn the words to the rap. Practice along side the song until the elocution is perfect.
Continue practice with the tongue twisters and continue to select rap songs with fast raps, but select songs with faster and faster raps. Build up the skills and tongue acrobatics over time.
Select your own beats and start trying to preform your own freestyle or planned rap.