It should be noted that getting there, and getting back will be two different routes.
To get there, just follow the signs to the Magic Kingdom. When you get to the MK Toll Plaza, stay right after you pass through. The road will fork, and then fork again. Stay far right on both turns. About a mile or two down the road, you will see the entrance to Fort Wilderness. KEEP GOING. There will be a SECOND entrance on the left. That is the one you want. Park there.
Currently, the Outpost bus stop is under construction, so there is a temporary bus stop set up in a closed section of the parking lot. Just look for the white tent, and you'll see the buses. You'll want to board any bus marked "SETTLEMENT" -- doesn't matter what the color is. Internal buses run every 10 minutes, and it'll take 5 minutes to get to the Settlement Depot, location of Pioneer Hall.
To get back to your car, board any bus marked "OUTPOST" -- once again, doesn't matter the color. Buses run from the Settlement Depot on the :10 minute mark.
Once back to the car, you'll want to take a LEFT out of the parking lot onto the main road. You'll pass a golf course on the left. At the traffic light, take a RIGHT onto Bonnet Creek. Continue past Port Orleans Riverside, French Quarter, and then Old Key West. At the end of the road, take a RIGHT onto Buena Vista Drive. You'll go past Caribbean Beach, and you'll soon see the Boardwalk.