It is a basic 2 to 1 ratio (sand to cornstarch)
6 cups of play sand (I used Crayola Brand – Blue)
3 cups of cornstarch 
1 1/2 cups of cold water
Begin by mixing the sand and cornstarch together. 
Next, add the water and mix it with your hands.  You will have to work it through until you do not see any more cornstarch (white). 
I store it in a plastic storage box and it doubles as the Moon Sand Sensory Center.
If it begins to dry out, just add a few tablespoons of water until it returns to the proper consistency.  
FYI – for the batch I took photos of – I did not have enough cornstarch.   It did not matter!  It was still great moon sand.  It is not an exact science!
 Add cornstarch to colored sand.
 Using your hands, mix the sand and cornstarch together.
 Add water.
 Mix water into sand mixture.
 Add space materials and play!