A shower mixer valve is attached to the stud wall frame behind the shower wall, and is manipulated by a hot and cold knob, or one dial. Its job is to mix the correct amount of hot and cold water, before it travels up to the shower head. The valve has two inlets on its bottom for the copper water supply lines. Traditionally, the hot line goes on the left, and the cold line on the right. To reverse the hot and cold, simply attach the hot line to the right inlet and the cold line to the left inlet.
Attach the mixer valve to a piece of horizontal wall stud, using 1 5/8-inch galvanised screws and a screw gun. Using the same size screws, attach the mixer and wall stud to the vertical wall studs on each side. Refer to the shower manual regarding the correct height and position of the mixer valve.
Run both hot and cold copper water supply pipes to below the mixer valve. Sand the ends of the two pipes, as well as the insides of the mixer's two inlets, using emery cloth.
Apply a layer of soldering paste (flux) to all of the sanded areas. Push the hot water supply pipe into the mixer valve's right inlet, and the cold supply pipe into the left inlet. Brace both water pipes to keep them in place, if needed.
Unroll 8 inches of solder, bending the last 2 inches 90 degrees. Turn on the propane torch. Heat one of the mixer's inlets on both sides. Touch the tip of the solder to the seam between the inlet and pipe. If the solder melts, apply 3/4 inch of solder around the seam. Carefully wipe away any drips of solder with a rag. Solder the mixer's other inlet in the same way.
Have a water spray bottle on hand in case of any fires caused when soldering.