Take a look over the available cars and the details of each. Once you've made your choice, tick "pay at station" or "pay now" (hint: it's cheaper to book online!) and click "book."
Choose whether you'd like extras like a sat nav, then scroll down and fill out the driver details section. If you have a flight number, type that in, otherwise move on to entering your payment information (if paying online) and tick the terms and conditions box. Click "pay now" once you're done. That's it, your car hire is sorted!