On your desktop
When you get to checkout, enter your discount code in the 'Gift card or discount code' field at the right, below your order summary. The total amount will automatically be deducted from your order.
On your mobile phone
When you get to checkout, click on 'View order summary' on top of the page and enter your discount code in the 'Gift card or discount code' field. The total amount will automatically be deducted from your order.
Attention: you can't apply a discount code with your subscription. Read more about it here.