You can enter a discount code in the last step of the order process. At the bottom of the checkout page you will see a bar with Discount code. Enter your discount code here and click on the Use discount code button. When the discount has been processed, the discount amount will be deducted from the total price. The discount amount is also shown separately.

Please note that each discount code has specific conditions that the order must comply with before the discount on the order is settled. These conditions are stated with the discount code.