Here's how I did it.
I used a 9x13 Pyrex dish that has a lid. Then I mixed together 1 box of baking soda and 1/3 cup of white hair conditioner.
That's all. Evelyn's been busy playing with it for an hour. When she's done I'll put the top on it and keep if for a week. If it starts to dry out, you can just add a bit more conditioner.