Who needs indoor plumbing when for less than $300 you can have a perfectly good outdoor crapper. Check your local laws.

Step 1: The Foundation

A good foundation is the key to a good outhouse. Dig a hole about 4' deep, 3.5x3.5ft square. Make it a good hole with even sides because you'll have to line it.
One point about soil. If you have hard clay soil, make sure that the drainage around the outhouse is good to avoid too much water getting in, because it won't want to leave (this could cause splashback).
You've got your hole. Drop a wooden box with tarpaper wrapped around it in the hole to keep moisture out. Level and even out the ground around the hole and place a foundation made of treated 4x4s around it. The foundation will for this one was 4'x3.5' (this allowed a 4x8 sheet of plywood to be cut at 3.5ft, one piece for the floor and the other for the roof with an over hang. 4'wide on the floor and 4' deep for the roof).

Step 2: The Frame

This pretty well shows the frame of the outhouse (nevermind the braces still on). It should be stable, but not too heavy since you may have to move it some day. I left the studs off the side walls.
Note the hole cut in the floor for the "business". I recommend coating the inside of the seat section with plastic to keep "it" from getting all over the wood after a curry night.
I sheeted it with 1/4 plywood and used 1/2ply for the floor, roof, and seat. I put my seat at 1'5" high, as you can see in the photo.

Step 3: Roof, Finishing, and Notes on Use

I shingled my roof in the standard manner. Note the vent pipe made of 4" PVC. An oversized cap is on top to keep water out. Holes were drilled in the end of the pipe to allow extra ventalation. Screen was wrapped around the pipe to keep the bugs out.
I would suggest that you paint the inside with a mold resistant paint like Kils. Additional windows can help with ventelation, just remember to put screen over them to minimize the number of bugs hanging out in there.
Also, one thing to consider if you think you may have a water flooding problem, place a cinder block upright in the pit right below the shitter. That way if it does flood, no splashback. Enjoy. Sorry for the poor spelling I'm in a hurry.