Use the piano to determine which progression of chords you would like to use in your rap production. Imagine that the chords you are playing are the bass line of your final song. Experiment with the piano in its lower pitch range in order to mimic the deep sounds of rap and hip-hop bass lines. This harmonic foundation will be an essential component of your final production.
Create a melody for either the verse or chorus of your production using the guitar. Imagine that the guitar is reflective of your own voice. Sing along with the pitches you are playing on the guitar and take note of any specific notes or registers that are difficult for you to reach. Avoid these notes when constructing your melody. Experiment with several different rhythms to find an attractive match for the beat you have created to accompany the melody. Find someone to play your new bass line while you play and sing your composed melody, checking to make sure that the two songs' elements match your artistic vision.
Record your guitar melody or piano harmony if you wish to sample the material within your song. As the process of sampling, or incorporating preexisting musical material, into rap songs has become an extensively debated copyright issue, use your own recordings whenever possible. Ensure that any material you record matches the tempo, or speed, of the new song.
Compose any additional melodic material with the guitar, such as catchy, rhythmic riffs or melody harmonizations. As mentioned in the previous step, you can use these guitar elements for sampling purposes. If you wish to use a different instrument to represent this material, experiment with your digital audio equipment to find a match that fits your aesthetic. As rap music typically doesn't incorporate these two acoustic instruments, explore the synthesized sounds of keyboard or digital audio workstations such as Logic, Reason, or ProTools.