To make baking powder:
2 part cream of tartar
1 part baking soda

To use homemade baking powder, substitute 1:1 for recipe quantity.

If storing add:
1 part cornstarch substitute, used to stabilize the activators

Substitutes for cornstarch:
1 part cornstarch = 2 part ap flour = 1 part potato flour, rice flour = 1 1/3 part tapioca flour = 2/3 part arrowroot, water chestnut flour

A great resource for understanding the applications of various thickeners is here, while a summary of baking soda leavening chemistry is here.