Download a free recording program. The Internet contains a wealth of free and open source programs, like Audacity, Ardour, Traverso, Wavosaur and more.
Download percussion loops (samples). A strong, electronic percussion is essential to a hip-hop beat, and you can download many free percussion loops free of charge from loop websites.
Download additional loops to enrich your beat. Depending on your personal flavor, you may want to add an orchestral melody line, an additional layer of percussion, a collection of sound effects, or a strong bass line. You can add as many loops as you need.
Open your digital audio software and arrange your loops on the software window. The audio editors mentioned in step 1 (as well as many others) use an arrangement called multi-tracking, wherein you can layer your recordings on top of one another by placing them on individual tracks, or horizontal rows on your track window. Drag your loop files directly onto the software window (each onto a separate track) and drag the regions along their respective tracks as needed.
Repair the timing of your regions. For best results, you should choose loops with the same tempo (speed), but in the event that certain loops play at a faster tempo than others, just click any region and select the "Speed" or "Tempo" option on your effects list and drag the slider to the left or right to slow down or speed up the region. Your effects list typically appears on your menu bar or on your virtual mixer.
Add your own instrumentation. Select an instrument sound from your list of "Software Instruments," "MIDI Instruments" or "Virtual Instruments" (depending on your software). If you own a MIDI keyboard, connect it to your PC and use it to play the notes. Otherwise, locate your "Piano Roll" or "Sequencer" view, usually accessed from the main project window and sometimes appearing as an image of a piano. Use the sequencer window to manually input music notes with your mouse. When you become comfortable sequencing your own music, you can create entire beats from scratch without using samples.
Save and export your beat. Use (File > Save) on your menu bar to save the audio project, and use "Export," "Bounce" or "Save as Audio File" (usually found under the "File" menu to save your beat as an MP3 or other file.