<<Disclaimer: Verify this information before applying it to your situation.>>

Sorry -  it took me a while to hunt down the recipe, and I never did find
the rest of the one I posted.  I did find a slight variation on it,
though, with more directions.  This comes from an old "Heloise" column.

2 cups baking soda (original post used salt)
1 cup cornstarch
1 1/4 cups water

In a saucepan, combine the baking soda and cornstarch and then add the
water (and food coloring, if desired).  Cook over medium heat, stirring
constantly until the mixture is like wet mashed potatoes.  Pour out onto
a plate and cover with a damp cloth.  Allow the mixture to cool, then
knead until smooth.  Use it immediately or store in a sealable plastic
bag  in the refrigerator.   To make into ornaments, etc. dry the
creations overnight or place them in a 250 degree oven for 15 minutes or
until dry.  Once dry, they can be painted and varnished to preserve.
If you just want a reusable playdough, I'm quite sure it will keep for a
while if tightly sealed and stored in the refrigerator.