If your store offers a rewards program then. You can do this in 2 simple steps:
1. Login in to your account using the email you used to place order/orders
2. Go to the homepage and click on the Rewards button and it will open up a pop up to show you your point balance as well as options on how to use them. You can choose your reward and them copying the promo code for that reward and use it at checkout.
Here is a Quick Demo Video