When you are ready to checkout, simply add the promotional code into the box in the checkout. If the code is valid, the relevant discount will be applied to your order. Please note if you are receiving an error to check the dates of the promotion or the products that it is to be applied to.
DESKTOP
If you're using a desktop computer, on the right hand side under "Apply Voucher Code" this is where you place your discount code BLACK and hit apply.
You will know this has been successful when there is a green messaging with "Discount has been applied" underneath. 
MOBILE
At the top under your purchase summary, drop down the "Show Order Summary & Gift Card" section and then apply the discount code BLACK in the field "Apply Voucher Code". It will appear with the green message "Discount has been applied".