Note: please be very patient when the map is updating itself. Look at the count sites on the map and find one that suits you; eg a site close to home or on the way to work. You can use the mouse scroll wheel to zoom in and out and change the map view type with the buttons on the top right corner of the map.
Click on the site marker you may want to book. Take note of its number and return to the previous page and enter the number when prompted to do so. Get in quick before your first choice is taken!