Sand all three pieces of wood, removing all splinters and rough edges.
Attach the 2-inch by 4-inch lengths of lumber to the wall, parallel to each other, with the bottom piece at the working height of the benchtop and the top piece 4 feet above it. Screw them into the wall at joists or use heavy duty screw anchors. Make sure these pieces are secured strongly to the wall.
Insert the two eye screws into the top of the top 2-inch by 4-inch piece of lumber attached to the wall, at each end. The eyes should be standing up vertically from the top edge of the wood.
String a chain through each eye screw.
Screw the bottom part of the hinges to the bottom piece of lumber attached to the wall, at each end and in the middle. Screw the top part of the hinges to the 4-foot side of the plywood.
Screw in the eye bolts at the left and right corners of the plywood that aren’t attached to the hinges. This is the side that is away from the wall when the benchtop is unfolded.
String each chain through the eye bolts and secure the ends of the chains with the carabiners close to the eye screws in the top piece lumber. Use the carabiner to adjust the chain so the benchtop sits level when opened.
Affix the hook and eye latches at the left and right ends of the benchtop, with the eyes screwed into the plywood and the latches screwed into the top piece of wood on the wall.