If you have a promo code you just need to apply it when you are checking out, on step 1. You will see a box that says: "Enter your code".
Please note that promo codes can't be used in conjunction with other promo codes, nor they work with sales items. Some can only be used once, depending on campaign.
Every time you receive a promo code the terms and conditions will be mentioned, wether you get them by email or phone.