In order to redeem a promotional code, you just need to make sure that:

1. The promotional is still active. They usually last just several days.
2. Your order is eligible for it. You just need to add the necessary products to your cart.

Then, in the Cart Page, under the last product on the list, you will find a box that reads "Do you have a coupon code or gift card?"

Just enter your promo code there and the promotion will be applied to your order! 

If you do not meet one of the two requirements set out above, the website will warn you that the promotion has not been applied because either it is no longer available, or you need to add other or more products to your shopping cart.

In case you have already placed an order and you forgot to apply a discount, visit this question here: How to apply a Promo Code after placing an order.