Hi James,
Thanks for your note and for moving my post! OK, So I'm probably about to reveal how non-constructiony my boyfriend and I are, but here goes!
First we followed the directions-- you are given these square plastic things that you put in the top back corners of the piece. First you put them inside and color in the hole. Then you drill the hole through the particle board on the back. Then you screw the plastic things in the back corners of the cabinet. For a "double wide" 47" piece, they suggest you do three-- one at each end and one in the middle support of the cabinet.
Then they say to simply scew the cabinet into the wall, using little washers that they provide. (If it will help, I can scan the directions tonight and show them to you.) This seemed suspiciously like not enough support.
So, my boyfriend thought it would be good to first put in those plastic sleaves. I measured as carefully as I could and he drilled them in. Then we put up the cabinet, making sure to the the top-left corner screw into the plastic sleave. The problem-- we didn't hit the other sleaves. And it felt quite wobbly.

We added some extra scews, but I still don't trust it!
I'm sure there's a smarter way to do this. We are working with dry wall that have metal studs. Can we drill through those?
Thanks for your help!!!
Suz