Apply plenty of bass and percussion. Whether you use live instruments, digital synthesizers or pre-recorded samples, you will want to ensure that each rap beat includes at least one steady drum beat (usually in the 4/4 time signature) and a deep, accompanying bass line. This will give you a foundation for both percussion and melody, providing adequate accompaniment for your rhymes.
Layer your beats with samples, also known as loops. If bass and percussion were all you needed, then every rap beat would sound remarkably similar. In order to give your beats dimension and originality, add some personal touches. Underground rappers as well as major producers and DJs incorporate pre-recorded loops (short sound files, typically four to 16 beats) like sound effects, keyboard melodies, vocal chants, arpeggios and other elements. Download appropriate loops from free loop websites like Looperman, Free Loops and Loopasonic, and add them to your beat using multitracking software GarageBand, Audacity, Pro Tools, Cubase, FL Studio or Logic.
Invest in a cheap USB MIDI keyboard for easy beat construction. If your stash of musical equipment is limited, you can use a USB MIDI keyboard to play the sounds of all the instruments you will need, including bass, keyboards, drums, orchestral instruments, strings and more. Just connect the keyboard to your computer, open a digital recording program like those previously listed, select a virtual instrument from your "Instruments" or "Voices" menu and begin playing.
Study the music of your favorite rap artists. Each subgenre, from crunk to break beat to old school hip-hop, has its own set of common conventions. If you want your own beats to closely follow the sound of a particular artist, listen carefully to the ways in which the artist's beats are constructed. Notice emphasis on melody, the approximate speed of the beats, the intensity of the drums, specific instruments used, and even subtle details like whether the beats incorporate a lot of echo, reverb or vocal harmonies. Practice constructing your own beats modeled after these styles.
Construct each beat so that it mirrors or complements the mood of the lyrics. For example, if you want to rap about throwing a party, use an upbeat tempo, bouncy melody lines and repetitive kick drums in order to suggest a party atmosphere and get listeners moving. If you want to rap about a personal tragedy, you might use a slower tempo, rooted in minor chords to suggest despair or frustration. Just experiment with different loops or note patterns and find the sequences that melodically convey the message you want to send.