We used about a 1/4 of a small applesauce cup of crushed cereal for each color of sand dough that we made. The amount of baking soda and water you use will vary depending on how much dough you wish to create.
Pour the crushed cereal into a container with some baking soda and stir to combine. My kids had mixed some cereal colors the day before, so our red has a few other colors mixed in. Slowly add water to the baking soda and cereal mixture. You will want to be sure to add a little water at a time and mix it in well. If you add too much at once it could cause the baking soda to dissolve. You are going for a texture that is somewhat moist and can be molded, but can also be crumbly.