Four 2-inch loop hooks*
Decorative chain* (based on your measurements)
*Available at hardware stores
Shelf How-To1. Drill holes in the shelf's two outer corners, 1 inch in from both edges. (The holes should be slightly smaller than the screw ends of the loop hooks.) Screw a hook into each hole.
2. Attach shelf to the wall, following the manufacturer's instructions.
3. Dangle a plumb bob -- or a house-key tied to a length of string -- from the ceiling until the tip just touches the loop hooks in the shelf. Mark the ceiling there and drill holes for screw anchors, tapping them in with a hammer. Screw in two remaining hooks.
4. Measure the distance from ceiling hooks to shelf hooks, and purchase (or cut) two lengths of chain to fit.
5. Hang the lengths of chain between each pair of hooks, and step back to revel in your own sleight of hand.