A floating shelf is one that appears to have no means of support. Shelf brackets aren't used, so the look is clean and contemporary. They can be painted to match your walls or left their natural wood colour. Make floating shelves by using a hollow core door and a wood bar, called a cleat, that's attached to the wall.
Buy an 18-inch wide hollow core door. When cut in half lengthwise, it will make 9-inch wide shelves.
Use a level to trace a line on the wall where your shelf will hang. Locate the studs and mark them with masking tape.
Cut the door in half lengthwise. Use a blade that will make a clean cut.
Measure the open space between the veneers of the door. This is where the shelf will slide onto the cleat, so accuracy is important. Cut cleats from a 2 by 4 to this width and the inside length of the door.
Drill 1/4-inch diameter holes into the studs where you'll place the shelf. Hold the cleat against the line you drew and drill through the cleat and into the stud with a 1/8-inch drill bit.
Use a wrench to install a lag screw into each of the holes. Use 1/4-by-3 1/2 inch lag screws and tighten them. Make sure the cleats are absolutely level.
Cut away at least 1 inch to 1 1/2 inches of the corrugated core inside the door. Scrape out any glue residue. Finish the shelf by either painting it or staining it.
Slide the slot in the door over the cleat. Make sure the floating shelf fits tightly against the wall. If it doesn't, use sandpaper or a plane to make it straight.
Remove the door from the cleat. Put carpenter's glue along the top of the cleat, and the bottom of the inside of the door. Slide the door back onto the cleat.
Nail the floating shelf to the cleat using 1-inch brads. Space them 8 inches apart.
Make shorter shelves by cutting the door to the size you want. Cut blocks of wood to fit inside the open cut at the ends.
Let the shelves dry overnight. Don't buy a door with holes already drilled in it for a lock set.