Assemble some reference images of snake fossils. Use several images to formulate the composition of your fossil sculpture. Decide how much detail you want to include and how large you want the snake fossil to be. Place the images around your work table for easy reference.
Knead 2 to 3 pounds of self-hardening clay like bread dough until it has a smooth, even consistency. Form the rough shape of the rock with your hands. Press it against real rock surfaces to create a realistic texture on the sides. Press the clay down on the table to create a flat surface on the top or side on which to sculpt the fossil shape into the clay.
Mark the dimensions and outline the fossil on the flat surface using the needle tool. Use it lightly, just enough to mark the surface. Follow these lines with a knife to etch the visible bone structures of the snake. Draw these features to resemble the structures you see in some of the reference images.
Experiments with different tools and objects on a separate piece of clay to try different effects before using them on the fossil sculpture. Detail the fossilized head of the snake. Press in and overlap sharp-edged, flat-sided objects to create the effect of overlapping bones and small flat surface areas.
Remove any tool or fingermarks with a damp sponge and let the sculpture dry for several days.
Create replicas of your fossil by pressing wet clay on the dry surface of your fossil sculpture. Keep the edges sharp around the flat area and texture the sides with rock. Slowly peel off the original to finish. The copies will look especially realistic, as they will be in relief, meaning raised from the flat surface.