@Rachel1446
it's easy, when you book accommodation you have to pass a few steps... I think on the 3rd step "Confirm and pay" you will be given the option to "enter your coupon code". You click on it and it takes you to the popup window where you can apply one of your coupons so you just click on "apply". The rest of the price (if any) will be charged for your credit card. If you book accommodation for less then your coupon is worth then you will lose the difference, you can't use your coupon for 2 or more bookings.
Tip: when using a coupon it is smart to contact the host before booking to make sure the place is available and you or the host will not cancel because in case of cancelation your coupon is lost.