Straightening a warped canvas by pinning it to a blocking board is the simplest, quickest, and most efficient way to reshape a finished piece of needlepoint.
A blocking board is a lightweight, pin-able board with a fabric covering that has been printed with a grid of squares. The grid is used as a guide for squaring the canvas during the blocking process.
Blocking boards can be purchased through many needlepoint suppliers. However, a towel covered pine board and a T-square will work just as well in a pinch (in desperation, I've even pinned canvases to a clean, carpeted floor.)
The only other supplies needed are heavy duty rust proof T-pins, a spray bottle containing clean water, and a steam iron.