I found 2 recipes that work well.
First one: Mix together 2 ½ cups of tap water and 1 ½ tbsp cornstarch in a saucepan (make sure there are no lumps).
Bring the mixture to a boil for 1 minute then cool to room temperature.
Fill a spray bottle. Store in the refrigerator.
When you bring it to a boil, the starch stays suspended and works perfectly: no shaking, no clogging, and nicely starched fabric!
Second one: Mix one part vodka to two parts water in a spray bottle (I use 1/3 cup vodka and 2/3 cup water).
Shake to combine.
The first recipe gives you a cloudy starch, the second one gives you a clear starch.