I just mixed up a batch today! My recipe is as follows....
3and 3/4 cups water
1tbsp. Corn starch
(mixed into the 1/4 cup cold water below)
1/4 cup cold water
Bring the 3 and 3/4 cups water to a boil in a pan on the stove. Slowly add the cold water/ corn starch mixture to the water and wisk until blended. Reduce heat and simmer 1 minute. Add 1 tbsp. lemon juice and let cool. After cooled, put into a spray bottle and it is ready to use. Shake before using if it has been sitting a while.
This makes a light starch mixtures as others have said increase the corn starch a tbsp. at a time until you get the stiffness you want. There are other recipes but this is the one I use.