I recently bought a completed unmounted needlepoint panel at an antique mall. It is a good size to make into a large flat seat back cushion. It is meant to be square, but it isn't: it must have become distorted as the person who made it worked on it. It is more trapezoidal; being off by as much as 3-4 inches. I would prefer not to cut it square. I have heard of the term "blocking" but don't know how to go about it. Thanks!