I have a much easier updated recipe. This is for a bulk amount of playdough, eg. if you are providing for a preschool class for everyone to share.
In a food processor mill 1 cup cheapest cooking salt until it is very fine. Add 500g rice flour, 300g cornstarch, 6 teaspoons cream of tartar and blend until nicely combined. Tip out into large pot (I use an 8L stock pot) - doesn't need to be on the stove. You don't cook this mixture!
Measure out 3 cups (750ml) boiling water and add 2 teaspoons of oil and a big squirt of food colouring.
pour boiling water mix into pot and mix mix mix.
It will come together quite quickly and once it really thickens you can start to knead it in the pot. It will be pretty hot still but I like the warmth on my hands. Knead for a bit until it is mixed smoothly and all the flour is incorporated. cover pot with some plastic wrap and the lid and once it cools down you will have a mega preschool class sized playdough batch that has taken about 5 minutes to make.