After you have added the desired products to your shopping basket, scroll to the bottom of the basket page and you'll see a box where you can enter the discount code. Click apply and your order will be discounted accordingly.
Some offers (e.g. 3 for £10 on selected e-liquids) register as a monetary amount discount and are applied in the checkout/basket page automatically. Any subtotal shown to you before this point will not include this discount.