Norma, I usually wing it because I've done this so many times. But here is a recipe I found on the internet: (this is for light starch that one can put in a spray bottle) Boil 3 3/4 cup of water. Meanwhile, mix 1 tablespoon cornstarch in 1/4 cup cold water and mix well. Add starch and cold water mixture to boiling water. Cook for 1 minute, boiling slightly. Let cool and put in spray bottle. Can add fragrance.
I generally put about 2 tablespoons of cornstarch in a pan (like one would use to make sauce), and wisk in about a cup of water. As it begins to thicken, I can add more water (and I always do) until I get it the consistancy I'm looking for (keep stiring to keep it smooth). If you want to use this for a very stiff stabilizer, then you'll want to leave it on the thick side.....or, if you want it to come out of a spray bottle you'll need to thin it down so it comes out the bottle spray tip.
Think of this as if you are making white sauce, and how you would want it cooked well enough for that. This mixture thickens as it cools. I'd suggest you just have a go at it...it's really easy to do. You'll soon figure out what consistancy works best for different situations. Shirley suggested adding a couple drops of rubbing alcohol to make it last longer....good tip! Otherwise, store extra in the refrigerator.