The non-AP-holder AP-info pages on disneyland.com do state to go to the main ticket booths to get the AP with monthly payments.
Just get the money together...you don't need to get it ahead of time, it wouldn't be activated and probably wouldn't have the right info on it to register it on disneyland.com anyway. If you get the full amount together, however, you could order the AP as an e-voucher, but that would be the total cost, not by payments...if you do that, you just take the printed voucher to the gates, hand it over to the CM, and they'll give you a ticket-shaped voucher that you would take to the AP processing center that day, to have a picture taken etc, and get your AP.
OR if you won't be going to the parks before checking into the hotel, you could add the cost of the AP onto your DLH room and pick up whatever it is they would give you (to take over to process at the AP center) when you check in.
Otherwise just do it at the ticket booths with the payments.