Using the Wah PedalThe wah pedal adds a funk sound to your guitar signal and brings it in and out with a "wah wah" type of effect. To use it, select it from the stomp box options and drag it into your stomp box array.
The wah pedal is modeled after a foot pedal which requires the guitar player to use their foot simultaneously while strumming the guitar. To use the GarageBand version, you have to click and drag 
the wah pedal's slider while the guitar track is playing.
Since you can't click and drag while also playing the guitar, you'll need to record the guitar track first and then add the wah effect afterwards.
Step 1: Record Track
Record a new guitar track in your GarageBand project.
Step 2: Adjust the Pedal Effects
Play back your track and listen for the parts where you want to add the wah effect. To add the effect to your audio, click and drag on the wah pedal. Sliding it up or down will adjust your audio accordingly.