You have a bunch of choices...
Take the Bus to MK, and catch the boat to Fort Wilderness.
Take the Bus to the Studios, and catch the bus to Fort Wilderness.
Take the Bus to Epcot, and catch the Fort Wilderness bus. *
Take the Bus or Boat to Downtown Disney, and catch the bus to Fort Wilderness. *
(* Note: For Epcot & DTD, you will also need to catch an internal shuttle to take you to the Settlement Depot)