I think you should block it before you sandwich it, to quilt it. Soak in the bathtub w/ a mild soap or Synthrapol. Blot and roll up in a large beach towel. Spread out on a flat surface you can pin into. I use the back of my foam design wall, made from pink insulation boards. Or some use their carpet, but lay down a clean sheet first. Pin (use sturdy pins or T-pins) to keep it in place and pull if you need to to get those wrinkles out. Let air dry. You can aim a fan at it to speed the drying. When dry, then sandwich and quilt it. I block many of my quilts that are meant to hang on a wall after they are quilted, before squaring up and finishing the edges. This makes them lay flat on the wall.