Open your drum machine or beat sequencer. Depending on what you're using, drums will be arranged in either a "Pattern Grid" or a "Sequencer." A Sequencer maps out the timeline of the song in a large grid, and is filled in like a sheet music staff, with spots on the grid representing drum hits. A Pattern Grid uses toggle switches to create short loops, which are then arranged in the desired order.
Start up your sampler or synthesizer to create a bass line. There should be numerous bass preset patches for you to start with; you can use one of these sounds or edit your own. Play the notes on a connected MIDI keyboard, or program them into the step sequencer, as with the drums.
Launch your sampler. Samples are a vital part of Hip-Hop, either as looped musical phrases used as a backbone to the piece, or as single hits or odd sounds used as a garnish, to create variation. You can load in audio files included with the sampler, downloaded from free sites, or cut from existing songs you like. Once the sound is loaded into the sampler, instances of the sound are triggered using a sequencer, just like the drum machine.
Familiarize yourself with the "Mute" and "Solo" buttons on your mixer, and how to automate them so your presses are recorded by the sequencer. The "Muting and Soloing" style of structuring a song is a throwback to the limited equipment available at the birth of Hip-Hop, but the style still persists to this day. Try pressing a few of the "Mute" or "Solo" buttons at the top of a 16-bar measure, and alter them on each repetition. This particular style is unique to Hip-Hop, and emulating it will lend your track real authenticity.