To use The Trybe AU promo codes, one needs to select the desired items and add them to the shopping cart. Proceed to the checkout page. Look for a field labeled 'Promo Code' or similar. Enter the promo code in this field and apply it. The discount should be reflected in the total purchase amount. Remember, each promo code may have its own terms and conditions, such as minimum purchase requirements or specific item applicability. Always check these details before applying the code. If the code doesn't work, it might be expired or not applicable to the items in the cart. In such cases, try using a different code if available.