Denny I've been known to do this frequently and I always use KERDI Mat with modified thinset. This way the piece can be constructed right-side-up.
Having recently done exactly what you are doing I set up a plywood work table. Screwed strips of lattice (in parallel) the width of the finished feature. In this case 6" X 48".
Then cut strips of KERDI Mat to suit the 6" X 48" need. Lay the KERDI strip between the lattice strips and go to tiling the on the KERDI. Remember to allow for your final grout lines when installing on the wall.
Once the piece has set over night, remove at least one lattice strip. You can now handle the flexible piece pretty effectively.
Apply thinset to the wall and judiciously place the finished piece to the wall.

Nothin' to it.
[Edited by Bud Cline Tile on Mon, Oct 26, 2009 at 8:02 pm CST]