Check the wall's stud locations to make sure they will match up with the vertical edges of the fiberglass tub. Measure the wall height and cut additional studs using a circular saw if the tub sides don't match. Slip the studs into the wall and toenail them into place using a hammer and 16d nails.
Measure the top lip of the tub to determine where the nailing blocks will be placed between each wall stud. Transfer this measurement to the studs and snap a line where the tub backing will make contact with the wall so nailing blocks can be placed.
Measure the space between the studs and transfer these marks to the lumber that will be cut for each individual nailing block. Cut the lumber using a circular saw and slip the nail block between the wall studs. Nail the block into place using 16d nails and a hammer.
Set the fiberglass tub against the wall where it will be secured. Check to make sure that the tub's back lip rests against the nailing blocks that were installed between each stud.
Secure the tub's outer lip to the studs and nailing blocks using a screw gun and 1 ½-inch drywall screws. Space each screw 4 to 5 inches apart around the entire area where the tub backing makes contact with the wall.