Measure your truck's bed. Ideally, the camper's footprint spans the area between the wheel wells and stretches from the back of the bed to the tailgate.
Build the camper base from 2-by-4 boards. Cut the 2-by-4s with a handsaw or circular saw to make a box to the dimensions of the pick-up bed, then use more 2-by-4s as joists spaced roughly every 2 feet. The truck's bed supports the weight placed on the joists, so they can be screwed directly the frame rather than using joist hangers.
Cut a sheet of OSB, or oriented strand board, to the size of your base. This sheet is the floor of your camper.
Attach the OSB sheet to the base with 2-inch deck screws. Use an electric drill to pre-drill the holes for the screws, then use a screwdriver bit to drive the screws through the OSB sheet into the base. Eight screws -- one at each corner, plus one in between each corner -- should be sufficient.
Decide on a height for your camper shell -- this depends largely on your own height. Think of it more like a wooden tent rather than an camper; this slide-on is used mainly for sleeping and storage, so you don't need to worry about building it tall enough to accommodate someone standing fully upright, unless that's important to you.
Plan the framing of the walls. Build them in two sections: lower walls as tall as the cab of the truck, and top walls taller than the cab. The top walls are built as a separate section from the bottom walls. This allows for easier construction and makes it easier to modify the camper later, if you think of something you'd like to add or change. For the framing materials, 1-by-2s are preferable to 2-by-4s because of they are lighter weight, and for the slight increase in floor space they provide, but the extra wall depth from the 2-by-4s might be handy for extra insulation if you live in a colder climate.
Frame the bottom walls and space the studs every 16 inches. Leave space for a door and try to keep it confined to the bottom walls -- this is much easier than building both the top and bottom walls around the door space. The size of the door depends on your size and physical abilities.
Use an electric drill with a screwdriver bit and deck screws to attach the walls to the base and to one another.
Frame the top walls. This process is the same as the bottom walls, except you want a 45-degree slope on the top wall that faces the truck's cab. This is to cut down on wind resistance when transporting the camper. The support studs for this angled wall can be cut using a miter saw set at 45 degrees. The top walls attach to the bottom walls in the same manner as the bottom walls attached to the base.
Frame a roof using the same and materials as the walls, 1-by-2s, and attach it to the top walls. If anything, use more decks screws than you think necessary; you don't want the roof to fly off while you're going down the road.
Build the door. Essentially, you're simply building a wall, putting it on hinges, and connecting it to the stud of another wall. It needs to be slightly smaller than its opening to allow for easy swinging; you might have to experiment a little bit with this before finalizing the door.
Panel the inside of your camper with OSB plywood or thinner paneling. Use screws to attach the panels to the camper's studs.
Insulate the walls. Cut strips of rigid foam insulation to fit between the wall studs -- make the fit as tight as possible. Once the insulation is in place, wrap the camper with a vapor barrier such as Tyvek house wrap. While not mandatory, this step increases the life of the camper by protecting it from rot, and it helps keep the interior cool during extreme temperatures.
Attach OSB plywood or thinner paneling to the camper's exterior. Whichever you use, seal it with waterproof paint to protect against rotting, as it only takes a few storms to ruin an unprotected piece of wood.
Caulk and flash the camper's corners and other joints. This keeps water from seeping into the camper and rotting the exterior from the inside out; the interior should already be protected by the house wrap.
