Tools: Utility knife and keyhole saw, level, plumb line, screwdriver.
1. After you cover the studs with wallboard, use a utility knife to cut a small hole over all dimensions on the outside of the face plate. Be sure you are cutting between the studs, not into them. Insert the brace section into the hole. Hold it in place with your hand as you bring the face plate up to it. Screw the front piece into the brace with the hardware provided by the manufacturer. The screw is designed to draw the brace and the face together, securing your installation.
2. If you want to install ceramic accessories in your tiled wall, simply open a space in the wallboard or mortar bed as you see fit and set the accessory as if it were an ordinary tile. If you want to install a metal dish in the wall, complete your tiling first. Be sure to leave a gap slightly smaller than the accessory and follow step 1 above.
3. If your wall is already opened up for extensive patching or some other reason, you can toenail a 2 by 4 between two studs, and use it as a brace for screws. Few accessories require such bracing, but it is useful for items like horizontal grab bars near the bathroom.
As a frame of reference, in a small bathroom a shower may prove to be so much more practical than a tub that you may very well intend to dispose of the larger fixture all together to save space.