Measure the shower base and installation area to make sure the base fits properly in the space.
Slide the shower base into position so the sides are flush against the shower walls. Trace along the sides of the base on the tile or flooring with a marker if the pan overlaps pre-installed flooring. Remove the base. Trim the tile or flooring you marked for a cleaner, level installation. Use a tile saw to cut the tile, and a razor to cut linoleum or vinyl flooring. Pry up the flooring with a pry bar to remove.
Attach the shower drain. Place the top half of the drain into the base drain hole from through the top side of the base. Thread the bottom part of the drain onto the threaded portion of the drain and tighten with a pair of plumbing pliers.
Check the floor for levelness. Place a carpenter's level along the length and width of the floor. Note areas where the floor drops below level. The pan will require shims in these locations. Place a shim in this spot, one that brings the floor to level. Mount the shim to the floor with a sheet rock screw and power driver.
Slide the shower pan back into place. Level the sides and floor of the pan. Slide additional shims under the pan where the base is below a level grade if necessary.
Mount the shower base to the wall frames with sheet rock screws. Drive galvanized screws through the base in the pre-drilled mounting holes and into the wall frames.