First, you'll need to follow my instructions for Installing Soundfonts (SF2) on Mac. Once your soundfont is installed, you can use it to play music in Garageband. Here's how to create a track with your new instrument.

1. Select the Track menu then New Track
2. When prompted, select Software Instrument and click Create
3. Double click on the icon of the instrument in it's track to open the controls
4. Click on the Show Inspector button
5. Click on the plug-ins section
6. Click on Instrument, AU Instruments, Apple, DLSMusicDevice, then Stereo
7. In the window that opens, select your SF2 file in the Sound Bank dropdown
8. Select the Window menu then Show Musical Typing to open the keyboard

You can also use the Audio FX option to add additional effects like compression.