Determine which type of truss bridge is suited for your situation. For a relatively short span, a king-post truss bridge will work well. Some other types include the queen-post truss, Howe and Pratt trusses--which work for spans of several hundred feet--and the Warren truss.
See if the design is more suited for a "through" bridge where the roadway lies at the bottom of the trusses and you drive "through" the trusses or a "deck" bridge where the roadway is on top of the trusses. The "through" bridge's members are mostly loaded in compression, while the "deck" bridge's members are loaded mostly in tension.
Choose the materials and connections. With a steel design manual, see which steel profiles and connectors will work best. If you will use tubular steel, you may want weld connections. If you're using "I" beam steel, you will most likely use bolt connectors. Some trusses are still made of materials such as wood but not many.
Prepare abutments. This includes excavating to bedrock and pouring concrete. Re-bar and anchor bolts will need to be placed into the concrete so you have something to attach the steel to.
Build your trusses. A truss is a series of triangles formed to take the compression load across the bridge. The simplest form of truss bridge is two right triangles with an upright in the center of your span and the hypotenuse stretching to the abutments, otherwise known as a king-post bridge. You will need two identical trusses, one on each side of the roadway.
Crane the trusses in place if they were not built in place, then install the cross members attaching the two trusses together.
Apply the bridge decking and paving. The material used here is determined by the use of the bridge. Many truss bridges are used for rail traffic, so they will be paved accordingly.