Block it and you should be fine. Get a steel carpenter's square and dampen the quilt --even getting it wet as Artsy suggested. Protect your carpet with plastic if need be, place a sheet on the floor and proceed to square the corners and stretch that rascal into submission. Allow to dry thoroughly before removing the pins (I use T-pins). Don't trim or cut it, just stretch it into shape. Measure as you go, making sure that the sides are straight and that the ends are straight. Good luck, but it should be fine.