Depending on the device you are using you have different ways of inserting the discount codes.
If you are using a computer you can insert the discount code on the "My Bag" screen below the "Proceed to Checkout" button or on the checkout process you have the "Coupon Code" box below the "Complete Order" button.
If you are using a mobile device you can insert the code by finding the "Show Details" button on the bottom of the "My Bag" screen. When you click this option you will be shown with the Order Details where you can insert the discount code.
On both options please remember to click "Apply" before proceeding with your purchase.