Mark the shelf board evenly using the pencil in each of the four corners, about ½ inch from each side. Predrill holes through the pencil markings, drilling the same distance as the length of the eyelet hooks. Use a drill bit which has a diameter that is the same or just smaller than the diameter of the eyelet hooks.
Insert the four eyelet hooks into the four predrilled holes. Screw them into place by hand or insert a screwdriver through the eyelet and rotate it clockwise to tighten the hooks into place. Continue tightening until the eyelet heads come in contact with the wood, then tighten until the eyelet hook is perpendicular to the shelf board. Rotate all four eyelet hooks to face the same direction.
Attach all four screwgate caribiners to the four eyelet hooks by opening the caribiners and passing them through the eyelet. Attach one end of a chain to one of the caribiners and tighten the caribiner gate securely. Repeat with the other chain, attaching it to the caribiner farthest from the first one.
Hold the shelf up to the rafter and pass one of the chains over the exposed beam. Attach the other end of the chain to the caribiner which is closest to that chain’s other end. Repeat for the other chain.
Place the carpenter’s level on top of the shelf and make sure that it is level. Shorten either chain if necessary by removing the last chain link from the caribiner and reattaching the second to last chain link in its place.