Turn your canvas around so the back faces you, and make a small pencil mark on each side of the wooden stretcher bars where you want to install sawtooth hangers.
Use your ruler to measure the distance between the mark and the top of the bar. Adjust the location of the mark until each is an equal distance from the top of the stretcher bar.
Use a hammer to lightly tap each of the four tacks into either side of both sawtooth hangers.
Hold the canvas up to the wall where you want it to hang.
Draw a light line along the top of the canvas using a pencil.
Flip the canvas over so the back side faces you and use a ruler to measure the distance between the center of each of the sawtooth hangers.
Use a pencil and a ruler to draw two marks on the wall beneath the first line you drew, which are a combination of the distance between the sawtooth hanger and the top of the canvas, and the distance between the center points of each sawtooth hanger.
Hold the ruler and the level up to the second two marks you drew on the wall and adjust the orientation of your ruler until the bubble in the level is in the center of level's view.
Adjust your markings so they fall along the level line.
Use a hammer to tap two nails into the two spots you marked with the ruler and level, leaving about a 1/4 inch of the nail head out of the wall.
Place your canvas onto the nail heads, so the canvas hangs from the sawtooth hangers.
Stand back and look at the canvas to make sure it looks level.