|We'll include some wall-mounting screws like this with any shelf or stair. (The screws you get might look a little different.) These screws are intended to mount your unit on half-inch drywall, which is what most people have. If you have some other kind of wall, please ask for mounting hardware at your hardware store.
You will need a pencil, a drill, a level and a phillips-head screwdriver.
First, put your cats out of the room. They'll want to "help"! Trust us on this point.
Next, decide where you want your shelves and stairs to be. You'll have to judge how far your cat can comfortably jump to reach each unit. Once you have a position, put the unit up against the wall and use the level to be sure it's horizontal. Then mark each of the holes with a pencil. There will be at least two mounting holes for each unit and usually more.
Drill a hole of about 1/4" through the drywall for each mounting screw, then twist in the plastic mounting anchor or tap it in with the butt of the screwdriver. Important: don't use the screws without the plastic anchors unless you're going directly into the stud, because they won't hold in drywall.
When the plastic anchors are in place, put the shelf back against the wall and put in the screws through the holes in the wood and into the plastic anchors. Important: you must screw the mounting screws all the way down, so that the wood backing of the shelf or stair is flat against the wall. If there's space between the wall and the wood, the shelf will not be stable.
That's all there is to it. If you would like more detailed instructions, you can find a good article at eHow.com