Sharon Schamber has videos on how to block blocks; you can also block an entire quilt top. Basically it means wetting it (Sharon uses a lot of spray starch), pinning it in place (you would need to stretch yours and then fasten with pins), and then ironing until the starch is dry. Starch will make sure the fabric retains its shape.
If you can steam iron the areas into shape, you can use spray starch at the ironing board. Spray starch can be tricky to work with. Sharon Schamber uses a pressing cloth with it; this helps make sure you don't scorch the starch with a too-hot iron.
If you don't quilt it enough, the fabric has an opportunity to spring back to its original bow. Sufficient quilting will keep the fabric stretched to where it needs to be to lie flat.