You can use your voucher once you have items in your shopping cart. Navigate to the shopping cart icon and click on Edit Shopping Cart. Here you can scroll to the bottom of the page to enter the voucher code.
Some users may experience an issue with displaying the voucher field. If you tick the box highlighted by the arrow below, the voucher code field should be visible.
With tick in the box:
Now you can enter your code. The respective discount will be applied to your order.