I have two of them. The one behind the stove was mounted to a board...and the thinset...don't laugh it works, is liquid nails....lots of liquid nails. He spread that like peanut butter on the board, then he layed the mosaic on the board and pressed it flat using a roller. After it was "down" he put piles of tile and granitedown on it carefully (don't slide them, stack them or use those nice cement blocks) all over it to make sure it was totally flat.

He had done a couple of them before so he knew what to do on this job. The one on the floor was done the same way, only no board was necessary because of it's location on the floor (it's a carpet in my entry hall). He said the first time he did this he tried all kinds of options but found the liquid nails worked best and they won't release under heat (behind a stove). We had unrolled ours a few weeks before and the tile was sitting on them to flatten them earlier, but they still had some lumps.

You might consider having him attempt this with a couple marble mosaic squares from Home Despot on a board just to get the hang of it. (that's how our guy experimented with his previous jobs...he told me because I have another I might put on a table).

It was grouted in place on the wall after it had been sitting to let the glue dry for a day or two (I don't remember). it's firm as a rock up there :)