Determine the length and width of the bridge.
Shovel and level the areas where the bridge will land.
Set two pier blocks on each side of the ditch so that the straps are spaced to the width of the bridge. Position the blocks so the straps are perpendicular to the ditch and the blocks are positioned directly across from each other and level.
Set the two posts between the straps on each side of the ditch. Adjust to make sure the posts are parallel to each other and level. Bolt the posts to the straps.
Screw the 2-by-4 inch boards across the top of the posts using the 3-inch deck screws. Space the boards 2 feet apart and use two screws on each side.
Screw the deck boards to the 2-by-4 inch boards lengthwise from one side of the ditch to the other, using the 2 1/2-inch deck screws. Space the deck boards about 3/8 inch apart and use two screws per connection.