Fold the 1-foot-by-4-foot paper in half horizontally to create a 1-foot-by-2-foot folding sheet. Begin at the top of the center fold, and create an arch, connecting the top center point to the bottom corner on the opposite side. Once you are satisfied with the arch, cut both sides of the paper along the line to create a full arch template.
Lay the template on the 1-foot-by-4-foot section of plywood, and trace the template. Cut the arch out of the wood, and repeat this step on the two remaining 1-foot-by-4-foot sections of plywood.
Attach two 2-foot sections of 4-by-4 to each plywood section. Measure 1-foot from each end and 18-inches below the edge and attach each 4-by-4 to the plywood. Nail the 4-by-4 into place with four nails each driven through the plywood into the 4-by-4.
Locate where you want the bridge to be placed, and mark the location for the four-by-fours to be planted. The arches should each be placed 18-inches apart, creating a total width of 3-feet.
Dig six post holes. Dig a hole for each four-by-four measuring 18-inches deep and 1-foot wide. Set the arches in place, and pour cement into the holes until the cement is ground level. Allow to dry for 24 hours.
Measure and mark 4-inches from each two-by-six tread end. Begin attaching the treads at the center point of the bridge. Leave the 4-inch overhang on each tread end. Place two nails into each arch location for each tread; six nails total per tread. Continue to attach all of the treads until the bridge is constructed.
On each end tread, place one wood baluster on each end. The baluster should be placed in the 4-inch overhang sections. Nail the baluster into place from the bottom up with two nails. Attach a baluster on each end of the center tread in the same manner.
Take one of the 3-foot sections of 2-by-4 and place the end on the center of the center baluster. Use two nails at each point of location. Extend the 2-by-4 to the end baluster, and nail it into place. Finish the stair rails by attaching the remaining three sections of 3-foot long 2-by-4s.