Depending on the option you choose you will either get a Will Call confirmation number or you can pay to have vouchers mailed to you. If you go with Will Call, you enter the confirmation number into
MDE. With a voucher, once it arrives in the mail you enter some number off the voucher into MDE but I don't know which one.
Once you get to WDW, take your Will Call confirmation page (or vouchers) to a Guest Relations window outside any park or at DTD. They will give you your actual passes. At that point you have an activated AP/PAP and the 1-year countdown starts that day.
In my case we had to hand over our Magic Bands to the CM as well and he had to do something with them. This was in the early days of MB testing so I'm not sure if that's still necessary. Just make sure you have them with you even if you opt to get your passes at DTD.