Whether you're signing up for a subscription or buying once, there will be a place for you to enter a coupon code.
To add a coupon code on desktop
When you're on the order summary page there will a place on the right hand side (as shown below) to add a coupon code; just enter it in the field provided and hit "Apply", and the discount should apply immediately.
To add a coupon code on mobile
Go through the checkout process and you will be able to add the coupon code on the Payment page at the end (as shown below).
If you're having issues with a coupon code, check to make sure the code has not expired or does not have specific parameters that you need to meet; you should be able to find this information wherever you received the coupon.
If you have checked this information and it's still not working, email us and we'll be more than happy to help you out!