On a computer - Click on your cart and then click on the ‘Checkout Now’ button.
You will then see a blue ‘Discount Code’ link below the ‘Order Details’ panel.
Click on the link to open a box to type your discount code into.
On a mobile view - Click on your cart to open it. The blue ‘Discount Code’ link will be below the ‘Continue to Payment’ button.
Click on that link to open a box where you can input your code.
Please note - Discounts cannot be stacked together to create a bigger discount.