Quite frankly it would be easier to make the drive and hang the mirror / picture/ piano or whatever than to write a step by step description on hanging a cleat system but here goes ONE: Locate and mark a level line where object is to be hung on wall, TWO: Pre-drill the wall side cleat for the number of anchors that is determined to be used, for a 40# mirror that is lets say 24" wide I would suggest 3 alligators. http://www.toggler.com/products/alligator/installation.php or sharkies: http://www.powers.com/product_07585.php One in the center and one on each side 3" - 5" inches from the ends, place the cleat at the the marked location and using a pencil through the holes mark the wall then set the cleat aside and using a 1/4" masonry bit drill through the plaster ( if you are lucky you could hit lath and wont have to use the anchors, just drywall screws ) insert the alligator and gently tap in with a hammer until flush to the wall (check the manufactures instructions). Using flat head screws tighten the cleat to the wall. THREE: Locate the other half of the cleat to the object being hung and attach it using an appropriate sized flat head screws and place your object just above the wall cleat slide it down and your done. Make sure that you understand the cleat system: http://www.google.ca/search?q=%22french+cleat%22&hl=en&num=10&lr=&ft=i&cr=&safe=images I have literally used thousands of the alligator wall anchors in all sorts of materials and as far as I know there was never any failures with them. Stud attachment isn't necessary for this hanging system... the plaster literally does the holding

You should have asked this question in Home Repair or Remodeling, but the correct way to attach a crossmember( I believe>to studs is by nailing. You could also use metal hangers in conjuction. Since you have used screws already, I think you would be OK. Next, you decide how high the bottom of your shelf will be and using a level make a horizontal line across your new drywall even with the bottom of the shelf. Next, decide how long a screw you will need to hang the shelf (thickness of the shelf plus the drywall plus depth into the crossmember you installed. Next, drill a 3/8" countersink hole on each side member, about 1/2" deep, in the shelf where you want to screw it to the crossmember you installed. Predrill the screw holes in the center of the countersink holes you just drilled (slightly less thick than the thickness of the screw) clear through the frame of the shelf. Now screw the screws in the shelf so about 1/8" protrudes. Line up the bottom of your shelf with the level line you marked on the wall, hold it against the wall and push it so the screws make a mark in the drywall. Remove the shelf and at those marks, predrill the screw holes through the drywall and into your crossmember. Now put the shelf against the wall again and drive in the screws so they are countersunk about 1/8" or more. Check for level. Now plug the screw holes with pieces of 3/8" dowel or some decorative plugs. As for the finishing work on the drywall, you can get a book at any homecenter that tells how to do that.

-to achieve the rated load capacity of the hooks you recently purchased, each hook must be nailed or screwed into a wooden or metal stud. Hardware stores sell stud-finders, for around 20 dollars. You skim the device across the wall surface, and it lights up when you are over a stud. Wooden studs will accept either nails or screws, and metal studs will take screws only.

Thanks, all. I am embarrassed to say that when I completely unpacked the mirror I found that it did in fact have D-rings and wire on it already. I thought that the reason it was packed up was that it had no way to hang--isn't that why I didn't hang it when I bought it last August? Isn't that why it was still covered in cardboard and tape 14 months later? Jiminy. So, the wire is taken care of, but I don't have a stud finder, so I am going to borrow my neighbor's tomorrow(if he can find it). Meanwhile, I'm faced with the challenge of deciding where I want to hang it. I planned to hang it in the dining room, but when I had my daughters hold it up there tonight, it was a disaster--I don't know why I didn't think through what it would be reflecting. An extra view of the pile of recycling stacked on the counter next to the back door? No, thanks.