Ensure you are familiar with all the parts needed. Many manufacturers sell kits to build the camera body; however, the bellows and lens may take special research to find. The Ebony is considered the premium ULF and is a good place to start researching your own design. There is a large community of photography fans online who maintain sites with tips and forums for discussing building ULFs.
Buy your materials. For a field camera you will need a large bellows and may be able to find that online in a kit form to purchase. You will also need a lens with an aperture that adequately covers the film you are trying to expose, several film holder and ULF film. Film in this size will be available only from specialty shops and may take a few weeks to obtain even if purchasing online.
Choose the materials for your camera box. Many want their UFLs to be as much of an art object as the images it produces and build intricate boxes out of hardwood. This depends on your budget and skill. Plywood and MDF are other options for the camera body. The body section is a simple box with an open front and back. Detailed plans for the camera box can also be found online.
Install the lens and shutter. The lens will likely be a fixed focal length in your camera that may or may not be interchangeable, depending on your camera. You will need to install the shutter mechanism carefully as it is the most crucial part of your camera. Once those are installed, attach the front rails and bellows to the front of the camera. Traditionally, ULF lenses are mounted on wooden plates that slide in and out; depending on how many lenses you have or intend to obtain, you may need to build several of these plates.
Install the film housing. The back of the camera houses the film and has a slot where you insert the film holder. Again, this is crucial so careful research and construction are necessary to avoid light leaks.
Ensure all edges are sealed and that the wood housing will not chip or crack. You can use varnish, Danish oil, polyurethane or clear acrylic lacquer.