Mix clay soil with sand and straw using enough water so that everything is coated with clay and you have cob. There is no set formula for cob. The amount of sand will depend on the type and amount of clay in your soil. Our soil comes from a small pond that was dug near the house site. The clay content is so high that we have to use slightly more sand than soil to prevent cracking when the cob dries. We are using about 5 parts coarse sand to 4 parts clay subsoil and then mixing in as much straw as we can. Straw plays a very important role in holding the cob wall together.
Our cob dries rock hard but can absorb surprising amounts of moisture. So long as there isn't enough water to wash the clay off the sand and straw the cob is stable. That's why we have a high foundation and will add an exterior plaster and a good roof. Last fall we stayed in a cob B&B in England that is over 500 years old.
As I write this on June 20th, we are just completing the second course of cob which brings the wall up to about 3'. I think we have done about 60 batches so far.