My friend Michael Byrne has posted an article that will be of interest to you.

http://www.jlconline.com/jlc/archive/kitchen/mortarbed_showers/index.html

I don't agree with everything he says, but you'll certainly get the idea.

Then I think you'll need to get over to a tile board, and I happen to have one.

http://johnbridge.com/forum/forum_toc.htm

John