Find out the towing capacity of your truck and measure the size of your truck bed. You'll need to know how much it can haul so you can plan for materials of the appropriate weight.
Assemble the frame, beginning with the frames of the two side walls. Connect them by installing the roof joists and the floor section. The back wall, typically only slightly more than a foot wide on either side of the door, is assembled last. Be sure to use rust-proof screws (such as deck screws) for the construction.
Install insulation and siding. For simple camper plans, your siding may consist only of plywood, with a plywood interior. Other more elaborate plans call for aluminum or fiberglass siding, which can be significantly more expensive. Often, opting for the lightest-weight materials can be the more expensive option. If you plan to have windows and a top vent (which most camper plans do) be sure to cut the holes for these features before installing the siding.
Install the windows, door and top vents, taking care to seal them for leaks using caulk or any other weatherproofing material.
Secure the corners of the camper body. If you decided upon aluminum or fiberglass siding, sealing the corners will involve caulking and installing some type of matching corner molding made of aluminum. If you're doing a strictly-plywood construction, you'll need to seal the top with a paint-on roofing tar, and seal the corners with weatherproofing. For plywood, you may also want to spend a little extra and install aluminum molding on the corners of the frame.
