This is from Diane Gaudynski's website.
Try my recipe for spray starch for all your pressing/piecing needs. Produces a super flat stable quilt: Dissolve a half or one teaspoon of regular Argo cornstarch (in your cupboard probably) in a few tablespoons of cold water in a 2-cup Pyrex measuring cup/pitcher. Boil 1 cup of water and stir into the cornstarch, stirring constantly. Add enough cool water to make 2 cups total of the mixture. It will thicken slightly and turn from chalk white to translucent. Let cool and use in a pump spray bottle. Shake it every time you spray. You may have to dilute it a little if it is too thick. Lasts a week or so, as there are no preservatives, no chemicals, no nothing that harms us or the environment, and it’s practically free, except for the spray bottle.