Follow the steps below:
Book your surf session, Water Park pass, Cable Park Pass or accommodation like you are going to check out using a credit card. Fill in all your contact details and check any necessary check boxes in the booking process.
Click 'Add to Cart' to take you to the next screen.
On the next screen above 'Contact' there is a line that says 'Apply a gift card'. Click on that line and enter your gift card:
Applying a promo code or season pass code?
These can be found on the first screen below your contact info fields: