The first thing you need to do is go to the service that the coupon is for, if your unsure you need to ask the person who gave it to you.
Once there you need to purchase the item, for instance if the coupon is for Maths Online 12-Months then you need to select the Maths Online 12-Months option and not the month-to-month option.
Click to go to checkout.
After logging in or creating your account the first page you will see is an order summary. Make sure everything is there then click "Continue"
You will now be on "Billing details", this is where you use the coupon. You will see this message, click it.
Once you have clicked the text it will expand and a coupon code box will appear, input your coupon into the box and then click the button "Apply coupon"
Your coupon will then be applied to your order and you will see it in your order summary at the bottom of the billing page.