Buy a set of eight stretcher keys, or stretcher bar keys, online or at any art supply store. Canvas manufacturers often provide a packet of wooden stretcher keys with their canvases.
Examine the back of your canvas painting and the wooden bars to which the canvas is stapled or tacked. These wooden bars are called stretcher bars.
Identify the two slots cut into the four inside corners where each stretcher bar meets another stretcher bar. Note that one slot is slightly lower than the other slot.
Remove four of your stretcher keys from their package and note that each key has a pointed end and a flat end.
Hold each stretcher key so that its longest side is parallel to the inside of one of the stretcher bars and slide it point first into the lower slot of an inside corner where the stretcher bars meet. Press each key firmly in place using your thumb. The Artists Blank Canvas website suggests using "a moderate amount of equal pressure."
Remove four more stretcher keys from their package.
Hold each stretcher key so that its longest side is parallel to the inside of the stretcher bar and slide it point first into the upper slot of an inside corner where the stretcher bars meet. Press each key firmly in place using your thumb, again with the same amount of moderate pressure on each key.
Check the canvas of your painting for tightness.
Gently tap the short, exposed end of each stretcher key with a small hammer to further tighten your canvas if necessary. Hold a small piece of wood between the end of the key and your hammer to protect the key and make striking it easier.