Play "Cargo Bridge" by clicking on the design icon in the upper-left corner of the screen, left clicking and holding on the nodes that appear in the terrain where your bridge should be, dragging the mouse out over the hole, and releasing to construct a bridge piece. Complete all the pieces of your bridge, and click "Test your bridge" at the upper-left corner to have the workers cross your bridge and complete each stage.
There are two types of bridge pieces you can build in "Cargo Bridge," wood walks and wood connectors. The wood walks form the platforms that the workers walk across to reach the other side of the bridge. The wood connectors are support beams that you must place to reinforce the bridge so that it doesn't collapse when the workers walk on it or drag materials over it.
A common wood connector formula to use on a basic two-piece wood walk bridge is to make a three-triangle construction of wood connectors above the bridge. Do this by making two triangles with points facing skyward directly on top of your bridge, stretching from end to end. This should use four wood connectors and looks like the letter "M" standing atop your bridge. Now connect a single wood connector to the two top points of your triangles. This uses five connectors in total and secures a basic bridge structure to accomplish the early stages.