I've been anxious for this for a long time...I finally have the quilt ready to cut the edges and face them. Because the piece is really long, I have put two tables together to accomodate the length. I have a cutting board that is 59" long which we laid on the sewing desk and then another 24x36 laid on the second table. To assure the long edge doesn't wobble, it is best to see that whole length at once. This is a ticklish business, and I have my husband to thank for helping me with it. (Carpentry skills and know how are very useful). Our approach was to cut one long edge; measure to the second long edge and cut that. For these long edges we use a metal straight edge purchased at a hardware store. It isn't see through like our normal rulers, but with care can be used for this purpose. Now we had 2 parallel sides and could use the large cutting mat to lay those sides along the lines. We then could cut one end perfectly "square". Then measure to the other end, flip the quilt around and cut that one square to the board...works like a charm.
Then to check our work, we measure the diagonals. Corner to corner, if they are the same we're successful...and they are!! Yes.