you can either buy them at the parks or online.
If you buy them at the parks, you can do it two ways: buy them outside the parks at the Walt Disney Studios Park-ticket-booth OR buy a one day-ticket online, use that to enter the park and get the annual pass at the Bureau Passeport Annual where the one-day-ticket-price will be subtracted from the amount due. In both cases the validity-period starts on the day you buy them (so when you are in the park).
If you buy them online, you get a voucher with which you can enter the park once and get your definitive annual pass made at the Bureau Passeport Annuel.
If you buy them online, you cannot be sponsored and get the 10% discount. If you buy them in the parks and have not had annual passes the last 2 years, you can be sponsored by another annual-pass-holder and get a 10% discount on your annual passes.
The passes, both fully paid at once or in monthly installments, are valid rightaway. Ofcourse, if you fail to pay the monthly installments, the pass will be suspended after some time.