Measure the length of the top edge of the painting.
Measure the distance from the top edge of the painting to the bottom edge of the top stretcher bar.
Decide where on the wall you want the painting. Using a level and a pencil, put a light mark at the center of the top edge where the painting will be.
Make a mark below the first one the same distance as the distance from the top of the canvas to the bottom of the top stretcher bar.
Use the level coupled with the measuring tape to draw a horizontal line as long as the width of the painting from the mark you just made.
Make a mark on your line a couple of inches in from the outside points of the line. You may need to move them in more for a large painting with thick stretcher bars.
Put a nail in the wall at each of the two marks you just made. For a heavy painting, drill a pilot hole for an anchor using the prescribed bit width, inserting the plastic sleeve, then the screw. Small, lightweight paintings can be hung with push pins.
Mount the painting on the wall, using the mark to center the painting.