1. First you need a Mac or an app for PC that can run Mac apps (don't ask me what ones, I have no idea).
2. Open GarageBand. Then choose new project and click on "Loops."
3. On the right side of the window, you can click on categories and types of instruments. Click on the song to listen to it. Then click and drag to put it in your timeline.
4. All the loops are programmed to sound great together and are the same tempo no matter what tempo you choose.