Measure the inside of your canvas's frame. On the inside left of the frame, mark a spot about a quarter of the way down the painting. Do the same on the right side -- measure to make sure that these markings are even.
Screw an eye screw into each marking. These screws often come packaged with the wire.
Measure a piece of wire approximately twice the width of your canvas. String it through the eye screws on either side so that an equal amount of wire sticks out from each one. The wire should not be perfectly taut -- allow about 1 inch of give in the middle.
Fold the wire back and wrap it around itself on either side. Wrap it tightly -- this makes it stronger and prevents loose wire from poking out and damaging your canvas.