Turn off the water supply to the bathroom area at the water main in your basement, or outside the home.
Remove any handle currently on the shower wall by prying off the cap of the handle and removing the screw. If there is no cap, remove the Allen screw from the base of the handle using an Allen wrench. Pull out the current cartridge by removing the washer from the end. Pull up on the retaining clip using the flat end of a screwdriver. Wiggle the cartridge back and forth using a wrench to remove it from the valve.
Turn the new Delta cartridge so the words reading "hot side" are on the left hand side of the shower. Align the end of the cartridge with the holes inside the valve and push the cartridge in.
Slide the included O-ring over the cartridge, as well as the bonnet nut. These two pieces just slide on.
Push the sleeve on to the cartridge until it fits tight. Place the handle over the sleeve and insert the Allen screw into the handle. Tighten the Allen screw down using an Allen wrench.
Turn on the water and test the shower.