Measure and cut the five 6-feet long 1 by 8s into 10 pieces with 3-foot lengths.
Measure and cut the 10-feet long 1 by 8 into three pieces with 3-foot lengths. You should now have 13 1 by 8 boards with 3-foot length made from the 6-foot and 10-foot long 1 by 8 boards.
Measure and cut four 2 by 4s to four 32 1/2-inch lengths and eight 31-inch lengths. You can cut two 31-inch pieces and one 32 1/2-inch piece from one 2 by 4, totaling 12 pieces.
Measure and cut the remaining 2 by 4s to four 44-inch lengths and four 40 1/2-inch lengths, totaling eight pieces.
Measure and cut the 6-foot 2 by 8 into two pieces with 36-inch lengths.
Mark the center of two 36-inch 2 by 8s.
Attach one 8-foot 2 by 8 to one end of a 36-inch 2 by 8. Line up the end of the 8-foot board so that it is flush with the end of the corner of the 36-inch board. You will be attaching the top of the 8-foot to the face of the 36-inch. Secure with two 2-inch deck screws in the top and bottom edge, from the opposite side of the 36-inch through to the 8-foot.
Attach the second 36-inch board to the bottom of the 8-foot board, in the same manner as the top.
Place a second 8-foot board in between the two 36-inch boards, at the center marks you made, and attach. Attach a third 8-foot board to the other end of the 36-inch boards, making sure they are flush.
Drill three 5/32-inch holes through the top and bottom 36-inch boards into the 8-foot boards. Your holes should be in a straight line. Secure lag screws and washers in each hole and tighten with the ratchet and 9/16-inch socket.
Use the carpenter's square to mark and cut the corners off one end of eight 31-inch boards.
Lay two 40 1/2-inch 2 by 4s and two 32 1/2-inch 2 by 4s out. Place the bottom edges against the long section of the frame to act as a guide stop. The tall 2 by 4s should be parallel.
Mark a width center line on the two 40 1/2-inch 2 by 4s. Mark a center line at both the top and bottom of the posts. Use a straight edge to connect the two marks together down the length of the two posts.
Attach one 31-inch board to two of the posts, lining up the edge, on the long side, of the 31-inch with the top edges of the 2 by 4s and the short edges of the board should line up with the center guideline you drew.
Mark 13 inches from the bottom edge of the board down the length of the 2 by 4. Attach a second 31-inch board, lining up the top end edges with the 13 inch marks and the ends with your 2 by 4 center lines. Attach with 3-inch deck screws.
Position the two of the 32 1/2-inch 2 by 4s on the left and right of your middle section. Each 32 1/2-inch should be spaced 21 inches from the taller 2 by 4s.
Place one of the angled 31-inch boards on the left side, on top of the short and tall 2 by 4s. Line the top edge flush with the top left corner of the 32 1/2-inch 2 by 4; the angled end should be butt against the end of the 31-inch attached to the tall 2 by 4. Secure with 3-inch screws at both ends. Repeat for the right side, connecting the middle section with the two side sections. The boards should be sloping downward.
Attach a second board to each side 12 1/4 inches from the top board.
Repeat steps 2 through 8 for the second rail. Temporarily attach the side rails to the 8-foot lengths of the frame with 2-inch decks screws, making sure the railing 2 by 4s are flush with the ends and bottom edge of the 8-foot boards.
Drill two 3/8-inch holes into each 2 by 4 through the 8-foot boards. Avoid hitting the lag bolts on the frame. Remove the screws and the railings.
Place the bridge frame in the location desired. Reattach the sides using the lag bolts, nuts and lock washers. Hammer the bolts through the holes if necessary.
Place the nuts on the end of the bolts and tighten with the ratchet and 9/16-inch socket.
Place the remaining 31-inch boards along the frame, in between the railings. Space the each board 3/8-inch apart to allow water to drain through.
Attach each board, using three 2-inch screws, to each of the three 8-foot boards. The first and last boards will also need four 2-inch screws in between the three securing screws.
Paint or stain as desired and allow it to dry.