Find the stud nearest the outside front edge of the tub by using a stud sensor. Measure 32 to 38 inches from the bottom of the tub and mark the spot with a carpenter's pencil or wax marker. The exact measurement depends on the height and preferences of the person who will use the grab bar.
Position the bottom bracket of the 18-inch-long grab bar at the chosen 32- to 38-inch mark and hold the grab bar in a straight, vertical position. Make sure the mounting holes are aligned with the stud. Mark the holes for both the bottom and top brackets of the bar with your marking instrument.
Find the two studs nearest the inside front end of the tub. They are usually about 16 inches apart.
Position the bottom bracket of the 24-inch-long grab bar about 6 to 10 inches from the top edge of the tub. Angle it so that it can be fastened to a stud at each end. Align the mounting holes with the studs and mark the holes.
Find two studs 32 inches from each other on the inside wall of the bathtub. One will probably be the second stud used for the angle grab bar. Measure 36 to 38 inches from the floor and mark the spot.
Position the 32-inch grab bar horizontally at the 36- to 38-inch mark. Align the mounting holes with the studs and mark the holes.
Drill all holes using a 1/8-inch drill bit. Anchors should be inserted wherever the stud is insufficiently deep to properly hold a screw. Install the grab bars in this position.