You can enter your discount code in step 4 of the checkout process under VOUCHER / GIFT CARD. Keep in mind that every voucher has it's own terms and conditions that must be met for the discount to be valid on your order.
After entering the voucher code and clicking REDEEM, you will either see a success message or an error message. In case the order doesn't meet the requirements of the voucher, it will indicate which requirements still need to be met.
After the voucher has been successfully processed, the discounted amount will be deducted from the total invoice amount.