Measure for your shower plumbing. The drain will already be cut out for you and some units have water knob installed but you will need to cut for the shower stem at least. Measure from the floor to the shower head stem and water stem, if necessary and transfer those measurements to the unit. This is definitely one of those measure twice, cut once situations. Cut in the wrong place and you can ruin your entire unit. Use a hole saw, and cut out the holes for the shower head and water stems.
Read the manufacturer's directions for preparing the floor for installation. Some require a foam insulation be spread on the floor before placing the unit in, while others just require a solid, level subfloor. Normally 3/4 inch plywood is fine and, if it is not level, you can use a floor leveling product to get it level.
Install the unit. Carefully place the unit so the plumbing goes through the holes that you've cut. Then drop it down over the drain pipe as it will be sticking up a little. Put together the drain assembly by placing plumbers putty on the bottom of the flange ring and putting it in place on the drain. Push down into the drain pipe and wipe away excess putty. Screw on the screen to the top of the drain.
Use sheet rock nails to attach the outside pre-drilled strip to the studs. Attach the shower head by placing Teflon tape over the threads of the stem in the same direction you will be screwing the shower head on. Put on the trim piece and screw on the shower head. Put the trim in place for the knobs and place knobs on the water stems by pushing them into place and screwing in the set screw at the bottom. Put a bead of silicone caulk around the trim of the shower head and knobs, and smooth out with your finger.
Seal where the bottom of the shower pan meets the floor with silicone caulk and continue caulking when the walls have been installed between the walls and the unit.