You can create ACID loops using the Chopper (although you can do it without the chopper too), you can record your own audio loops, or you can record your own MIDI loops. I find loop creation to be quite easy which is one of the reasons I use ACID Pro.
What I do is record a new MIDI track and play my part several times. Once I think I’ve captured enough good takes, I go back and listen to the track and highlight the measure I like best and choose Select in Chopper. I listen to it looped in the chopper to make sure it loops smoothly. If it doesn’t I’ll open the MIDI editor and tweak the note positions until it does. Then I select Chop to New Track… and I save the final loop. If I want that as a wave (*.WAV) file I just solo it and do a Render As… using Render Looped Region Only. If you started with an audio loop there is no need to render. It’s very easy one you’ve done it a few times.
