To use ParkSleepFly promo codes, follow these steps: 1. Select the items to purchase on the ParkSleepFly website. 2. Proceed to the checkout page. 3. On the checkout page, find a field where the coupon code can be entered. 4. Paste the promo code in this field and click Apply. The discount should be applied instantly to the total. Remember, each promo code has its own terms and conditions, so ensure the code is valid for the items being purchased.