There are two ways. One is to just book a DVC room through the Disney's central reservation system (CRO) applicable to all hotels at WDW. The other is to go through DVC member services (must be by phone, not online) and ask for a cash reservation for which the price will be 75% of the regular (non-discounted) price of the room through CRO. Note that as to which of those two is less expensive can depend on whether Disney, through CRO, is offering room discounts (or special discounted room and ticket packages) that apply to DVC resort rooms, e.g., sometimes what you can get through CRO is better than the 75% room only price through DVC.
As to when you can get the room, that varies. If going through DVC, you do not have to wait list first or even attempt to use your points first. It cannot be done through DVC more than 11 months out and it is typical for nothing to be available, if at all, until closer to the desired dates, and there is not set rule or practice that tells you when, before date of arrival, you may actually be able to get a room through DVC.