If you are hoping to apply it to your most recent purchase, you're out of luck. It isn't retrospective.
The offer should show the terms & conditions. Often they apply to purchases over a certain amount and you have to use the voucher code at the time of purchase, so you may be able to use it for the next thing you buy. Look to the dates it is valid though, the vouchers usually have a limited time.
If it is an ebay offer, pop whatever you want into the cart then before you finalise things, you should see a box that asks you to enter any voucher code. Once you do that, it should adjust the payment you owe and then you can go ahead and pay as usual.
