The first step is to determine what kind of birds you want to attract because that will determine the size of the box and the size of the entrance hole. Only cavity nesters will use a next box. Here are some plans that might work for you.
Be sure that your box has drainage holes and that one side is removable for cleaning. Building materials should not include pressure treated wood. The wood you use should not be painted or stained. Natural is better.
The nestbox mounting system is equally as important, and perhaps even more so, than the nestbox itself. Mounting the nestbox on a tree trunk, a wooden post without a baffle, or on the side of a building transforms your nestbox into a fast food restaurant for raccoons, cats, and snakes.