Turn off the water in the home by closing the main water line near the water heater.
Remove the old faucet handles by unscrewing the set screws on each faucet with a screwdriver or Allen wrench and pulling the handles off. Remove the faucet stems on each handle with a bath socket wrench -- just place it on the stem and turn counter-clockwise to remove the stems. Take out the seats for each handle with a stem wrench by inserting a seat wrench into the seat and turning it counter-clockwise.
Cut out the middle section of the water pipes, where the old double handle faucet was installed, with a grinder . A disc grinder with a metal abrasive disc is the tool to get this job done.
Fit the new shower body valve single handle faucet you plan on installing into place between the two pipes where the old faucet was installed. Connect the new shower body valve to the old pipes and sweat the connections by heating them with a blow torch. Solder the new shower body valve into place with plumber's solder and a blowtorch. Heat the seams of the valve and the pipes with the blow torch, and touch the solder to the seams to fill them all the way around.
Install the new shower faucet by inserting the faucet cartridge into the shower valve. Place the new trim over the cartridge and screw it into place using the provided trim screws in your faucet installation kit. Screw the faucet cartridge retaining nut over the cartridge. Slide the handle onto the faucet and secure it with a set screw. Alternatively, follow each step as instructed by your owner's manual. Each faucet is different, so the steps to complete the installation will vary slightly, depending on which single handle model you choose.