A GoodRx coupon works just like any coupon you take into the grocery store. At the time of checkout, hand the coupon to your pharmacist and save off the retail price of your prescription.
When you go to the pharmacist to pick up your prescription, let them know that you have a coupon before they scan your medication to begin checkout.
Or, if you're bringing a paper prescription to the pharmacy, present the pharmacist with your GoodRx coupon along with your prescription. This way, the GoodRx coupon will be automatically applied when you pick up your medication!