Shower mixer valves are attached to the stud wall behind the shower wall, and are operated by either two hot and cold shower knobs, or one dial. The mixer has a hot and cold copper water supply pipe entering two inlets on its bottom, and one copper pipe travels from an outlet on the mixer's top up to a brass fixture that holds the shower head and arm in place. The mixer's inlets, outlet and shower head's brass fixture all have to be soldered to the copper pipe.
Remove the shower wall, if it has already been installed. Turn off the water at the main shut-off valve.
Attach the mixer valve to a horizontal piece of wall stud, using 1 5/8-inch galvanised screws. This wall stud will be attached to the vertical wall studs on each side of it. Check with the shower instructions regarding the required height and position of the mixer valve.
Attach the brass fixture for the shower head and arm also to a vertical wall stud--again, check with the shower instructions for the proper height and position. Use 1 5/8-inch galvanised screws.
Run hot and cold water supply pipes up to the bottom of the mixer valve. Attach the pipes to the existing water lines with "T"-shaped couplings. Mark each hot and cold pipe at the location where the new pipes will be attached, and a second mark 3/4-inch from the first, also on each pipe--use a felt tip pen. Place a tubing cutter around one pipe. Lower the blade onto one mark, and tighten. Rotate the cutter once. Tighten and rotate again. Continue until the pipe is cut through. Now move the cutter to the second mark, and cut the pipe there also. Place the cutter on the other pipe, and cut through the pipe at both marks.
Sand around all cuts on the two pipes, as well as the insides of two "T"-shaped couplings. Use emery cloth. Then sand the ends of two new pieces of pipe. Apply soldering paste (flux) to all of the sanded areas. Push the couplings between the cuts on the two pipes, and push the two new pipes into the remaining ends of the couplings. Now measure, cut and install new copper pipe to the two already installed pipes, so that they run up to the inlets on the bottom of the mixer valve. Sand and apply flux to all pipe ends and couplings, before fitting together.
Sand the inside of the two inlets on the valve's bottom, as well as the outlet on its top. Push the two pipe ends into the two inlets. Measure and cut a new piece of pipe that fits between the valve's outlet, and brass shower head fitting's inlet. Sand both ends of the pipe, as well as the inlet on the brass fitting. Also apply flux to the sanded areas. Push the pipe into the valve's inlet and the brass fitting's outlet.
Turn on the propane torch, and unroll 10 inches of solder--bend the last 3 inches. Heat both sides of one "T"-shaped coupling. When the flux sizzles, touch the solder tip to one of the coupling's seams. If the solder melts, apply 3/4-inch of solder around the seam. Now solder the coupling's other two seams. Wipe any excess solder away with a rag--it will be hot. Now solder the other "T"-shaped coupling, as well as all other couplings. Then solder the valve's two inlets and outlet, and finally the brass fixture's inlet. Wait for all couplings to cool, before turning on the water.
Keep a water spray bottle close by, when soldering.