Remove any materials necessary to expose the subfloor.
Measure the opening at the threshold. The threshold should ideally be even with floor joists and wall studs.
Transfer the threshold measurement to 2-by-4-inch boards and cut three lengths of 2-by-4s to this size.
Stack and attach the three 2-by-4s to the floor joists and wall studs at the shower threshold using 16d galvanized nails.
Measure the spaces between the wall studs within the shower walls.
Cut lengths of 2-by-10-inch boards to fit the spaces between the wall studs.
Install a 2-by-10 in each stud bay around the shower perimeter to form a solid wall around the base of the shower where the pan will be created.