Open GarageBand on your computer. Click the "Track" button and then select "Software Instrument" option. Continue creating "Software Instrument" tracks in GarageBand until you have the total of number tracks you plan to use for the song.
Click and drag a MIDI file from a folder on your computer to a "Software Track" in GarageBand. Continue dragging MIDI files into tracks in GarageBand. Make sure you drag the MIDI file to the track you want it on. For instance, you should drag multiple piano MIDI files to the same GarageBand track.
Click "New Track," then "Software Instrument" in GarageBand. Look to the instrument list on the right side of the application. Choose a general instrument type and then select a more specific instrument within that category. For instance, you might select "Drum Kits" and then select "Hip Hop Kit." Continue this process for each software instrument track in your project.
Click the "Tempo" button at the bottom of the application. Click and drag the slider to set the song tempo.
Click and drag each MIDI file to the location in the song to the point you want it to play. GarageBand lists the song measures above the musical tracks to help you line things up.
Loop a specific MIDI file by clicking your mouse cursor on the upper right corner of the file and by dragging the file to the measure where you want the part to end.