You have to make a frame, something the general shape and then cover it with lots of wire. (his tummy is a tire)
There are lots of different tufa recipes, depending on the application, but what I use is:
1 Part Portland cement (note it has to be Portland cement, NOT concrete)
1 Part sifted peat moss (sifting is important, you don't want chunks)
1 Part perlite or vermiculite
Caution: Do NOT breath in cement powder when measuring out the portland cement, wear a mask. Do not handle with bare hands. Wear heavy duty gloves.
Mix with water. You just have to get a "feel" to know the right consistency.
Here was the form for my sea serpent:
Then it has to dry slowly, like any cement mix (like a driveway). Mist frequently or cover the first couple days with plastic, then keep misting for a week. After it cures (6 weeks) you can seal with Thompsons water seal.