log in that website, update your profile, click on LOCKER on the top, select the aera you want your locker to be, select the locker you want (you better go to the please and find the number of the locker you want, put a lock on it to PHYSICALLY reserve it), click on reserve, then make payment.
if you have an account before, there will be a history of anything you have done with the parking department, so if you want the same locker, you can easily find the number, and reserve.
gook lucg.