If you have had the misfortune of injuring yourself or breaking something because the lid on your desk or heavy wooden box suddenly fell back without warning, you need a chain support. Besides keeping fingers and other precious objects out of harm's way, a chain support -- or "stay" -- for the lid adds a nice finishing touch. Installing a lid support requires only a few minutes of your time and some basic tools.
Open the lid of your box or chest. Place the triangular carpenter's square at a 90-degree angle between the lid and the top edge of the item. Position one flat edge of the square on the lid, and the other on the lip, on the same side of the box or chest.
Mark the lid with a pencil at the spot where the highest point of the triangle touches it. Mark the lip of the box at the opposite point of the triangle in the same manner.
Mark the box on the inside, just below the lip, using the mark on the top edge as a guide.
Place one of the decorative chain mounting plates (with the chain attached) at the marked point on the lid. Stretch the chain across to the opposite mark inside the box to test the length. Adjust the mounting plates up or down to accommodate the length of the chain, while maintaining a relatively stable angle between the two points.
Mark the final position of the mounting plates on the lid and inside the box or chest.
Attach the mounting plates to the lid by inserting the provided screws through the holes and screwing them in with a screwdriver. Repeat the procedure with the mounting plate on the inside edge of the box.