I would block it. Put a towel on the floor (over a carpet). Pin the quilt down on it, angling the pins outwards, so that you can put it under a little bit of pressure, stretching it outwards gently. Make sure you pin it so that it's square (do that by measuring diagnally and making both the same). Pin all the corners down, and put a few along the straight edges. Spray it all with cold water, so that it's damp (not soaking wet), and leave to dry overnight. If you have pets, then shut the door to keep them out for the night. When dry, you can remove the pins and it will be flat, with no curly corners.