Determine the towing capacity of your truck by consulting your owner's manual. Keep in mind that for safety reasons, you only want to haul about 80% of the truck's total towing capacity when the truck is full of gear and people. Also measure the width, length and height of your truck bed, taking special measurements around the wheel well area. Since trucks are often different sizes, you'll want to know the exact size of your bed to create the proper box to fit in it.
Purchase do-it-yourself camper plans from a site such as Glen-L or Butler Projects. These plans have detailed instructions for many sizes and types of camper, to give you some flexibility in expense and time commitment. Read your plans carefully once you get them.
Buy your materials, and assess whether you have the proper tools. Most camper plans include materials such as plywood, 2-by-4's, insulation, siding, doors, windows and hardware. The plus about using pre-made plans is you don't have to guess how much hardware you'll need -- and the expenses are already laid out for you, making it easy to stick to budget. If you don't have a drill, saber or circular saw, a level and measuring tape, get them now.
Build the camper frame by assembling the walls, attaching them to the floor, and assembling ceiling joists and supports. Leave spaces for your windows and the rear door. If you're doing a cheap version of the homemade camper, you'll likely be building your camper right onto the truck; if you're spending a bit more and going for a more professional look, you may be able to build the thing in your garage and attach it later.
Place insulation in between the frame pieces and install siding over top. You may be using fiberglass or aluminum, or if you're going cheap -- simple plywood siding. If you are going for the cheaper version, be sure to take into account the weight of the various materials, so that you don't go over the appropriate towing capacity of your truck. Lighter-weight materials, like aluminum siding, are often the cheapest option, so if weight is an issue, this may be the way to go. As you install the siding, be sure to cut your holes for doors and windows.
Place and caulk your doors, vents, and windows. Take extra care to install plenty of weatherproofing around cracks that could lead to the outside of the camper.
Seal the corners and any exposed areas of the camper with weatherproofing, and install trim where appropriate. If you're using aluminum or fiberglass siding, it should act as an adequate barrier for rain on the roof; if you're using plywood, however, you'll need to seal the top area with roofing tar or paint.
