IMHO, the easiest way is to purchase on site at any Guest Relations window - the one in Downtown Disney MarketPlace is especially easy. All you will need is your
DVC membership card and a photo ID.
If you don't want to wait until you are on site, you can get a voucher (to exchange for the actual pass) either online via the DVC Members' Website or you can call MS. Both are quite easy. You can either have the voucher mailed to you (for a $3/order mailing charge) or you can opt for "will call". Either way, you will need to go to a Guest Relations window - either to exchange the voucher for an actual pass or to pick up the actual pass. You will need to show your DVC membershop card and a photo ID to get the actual pass.
If you also plan to purchase the Disney Dining Experience (DDE) card, do not choose the will call option. You will need the numbers on the back of the voucher or the actual pass to order the card.
Best wishes -