Download the iLife package, including GarageBand, if you don't already have it on your Mac (see Resources).
Open GarageBand. Create a new session by clicking “File” and then “New.” Select “New Music Project,” name the project, and click “Create.”
Add music tracks by clicking, dragging and dropping the music files into the GarageBand edit window.
Adjust the volume of each track accordingly by moving the track volume slider.
Pan each track left, right or center by scrolling the pan knob accordingly.
Click to highlight a track, select the “View/Hide the Track Info” button, and then click the “Details” arrow to add effects to a music track. Choose from a variety of presets or manually adjust each control to achieve your desired sound. You can also add multiple effects or reverb, compress, or equalize your tracks.
Begin the mixing process by panning the snare drum, kick drum, bass guitar or keyboard bass, and vocal to the center. Guitars, keys, tom-toms and cymbals are usually panned left and right.
Add compression and equalize your tracks. Compression, which helps control dynamics by making loud sounds softer and soft sounds louder, can smooth out individual performances. To use equalizers, boosting or cut the high, mid, and low frequencies until you're satisfied.
Give your tracks a sense of spaciousness by adding reverb. You can adjust this effect's room size, wet level and dry level to achieve the desired reverb effect.
You can insert other effects to achieve various sounds, too. For instance, you can add a tremolo or flange effect to a guitar track to give it a unique sound, or you could add distortion to a vocal to give it a low-fidelity sound. Overall, the best way to find out what effects work on which instruments is to simply experiment.
Click "Share," "Export Song to Disk," and then name the GarageBand mix. Click "Save" when finished mixing the music. This will create a stereo mixdown of the combined music files.