Measure the height of the tub, from the base to the top of the tub rim. Measure the width and length as well. Mark the halfway point of the tub's width on the floor. Draw a center line, the length of the tub, on the floor. Draw a vertical center line the same distance from the side wall.
Draw a horizontal line, across the vertical centerline, at the height of the tub's rim. Draw a horizontal line four inches above the tub rim line, across the vertical center line. This line indicates where the spout stub should protrude the wall.
Draw another horizontal line, to four inches of either side of the vertical center line, six inches above the spout stub line. This line indicates where the hot and cold lines should stub out. The diverter will stub where this line intersects the vertical center line.
Measure 65- to 76 inches off the floor and draw a final horizontal line across the vertical centerline. The line intersection will indicate where the shower stub should protrude through the wall.
Measure 10 inches from the stub out of the wall, along the floor, and draw a line perpendicular to the centerline to indicate where the drain access should be installed on the floor.