The classic outhouse design is a clapboard shack with a peak roof and a wooden door with a half-moon cutout. This is by no means the only possibility. Outhouses have been made out of river rocks, cinder blocks, bricks and playwood. The look of your outhouse depends on the effect you want to achieve. You may wish to match the other structures on your property, like Thomas Jefferson, who built one out of the same red brick, and with the same design, that he used to build Monticello.
"Prettify" your privy by painting a mural on its exterior, carving designs into the wooden door, hanging floral baskets from the sides or planting flowers around the base. Install a transom window over the door made of glass or a carved wooden screen. If you want to inject a little humor, paint the outhouse red with white trim, like a barn; add plywood pieces shaped like rocket fins or castle turrets; or paint your loo to resemble a telephone booth.
A well-planned backhouse should take thought for the details. If yours will be a functioning privy, and especially if it will be used by visitors, such as campers or hikers, install a roll holder, a book shelf or magazine rack, a strong latch and some modern-day equivalent of the "chicken switch" -- a switch that can be used to swat away insects and varmints.
If your outhouse is merely a lawn adornment, its placement is up to you; however, if it's meant to be used, it's important to take thought for where it's placed. For example, it's wise to place a working outhouse on a level area away from tree roots and rocky or uneven ground to help prevent users from tripping and falling, especially at night.