To build a garden bridge, start by determining the width of the bridge and buy deck boards of that width and then design it. Afterwards, choose where you'll sink the concrete blocks, two on either side that will form the ends of your bridge. Dig pits for the blocks. Tightly place the concrete blocks to form the foot of one end of the bridge. Place the level over the concrete blocks, and tap the blocks using the butt of a chisel or trowel until their tops are level. Put the other two concrete blocks on the other side of the bridge ensuring that they are parallel to the first two blocks. Ensure that these blocks are also level on the surface. Next, put the two 12-foot posts across the ravine, ditch or water body to rest on the concrete blocks on either side. Drill holes into the concrete blocks with the masonry bit. Drill holes at the ends of both the posts. Fix them together with the lag screws and test for strength. Mark where you'll place the deck boards using the measuring tape and carpenter's chalk. Fix the deck boards to the side posts using the rust-resistant deck screws ensuring that they cover the entire length of the bridge. Again, mark the places in the side posts where the vertical posts of the side rail are to be attached. Attach the vertical posts of the side rail to the side posts at suitable intervals. Lastly, place one horizontal post to run the length of the side rail at the top of the vertical post. Screw all the timber simultaneously using rust-resistant deck screws.