Upon checkout you should see the following text box to enter your promo code in:
Then press apply and you'll see the discounted amount listed above your new total.
If your code isn't working, please check that your code doesn't have any characters like quotation marks or punctuation.
Another reason your code could not be working is if you already have a discount added. We only allow for one discount at a time.
Lastly the promo code could not be working if it is a code that is specific to products. For e.g. a 10% discount on blankets will not apply to the mask.