The GarageBand stomp boxes are modeled on the physical stomp box pedals guitarists use when playing. These boxes add different kinds of effects to the guitar's sound. Whereas the physical ones are boxes with a pedal switch used to turn them on or off, the GarageBand versions are virtual boxes with knobs and buttons you can adjust by clicking them with your mouse.
First click on the Edit button in the top right corner. The stomp box options will appear below the amplifier image.
Then select the stomp box that you want to use. Click and drag it into your stomp box array.
Adjust the Stomp BoxTurn on the stomp box by clicking the On button at the bottom. You can adjust its settings by dragging the knobs up or down.