Take out the screw holding the shower-valve knob in place. It is usually a Phillips head screw in the center of the knob.
Try pulling the knob directly away from the wall to remove it. This rarely works the first time because the knob is often stuck.
Use a gear puller on the knob if you have access to one, and the knob will come off easily. If not, hold a 1-inch-thick ("1 by") piece of lumber up against the shower wall. Slide the claws of a hammer behind the knob and pry it toward you, using the piece of lumber as the surface you pry against. The knob should come off with a little elbow grease.
Take the threaded metal sleeve off the shower valve with a wrench.
Use the razor knife and scraper to cut away any caulking that is blocking your access to the shower valve.
Insert a wrench onto the shower valve, and turn it counterclockwise to loosen the valve. Unscrew the valve by hand, and you are finished.