In a medium size mixing bowl, combine the plaster of Paris with water in a 1 to 2 ratio, which is one part of plaster of Paris to two parts of water.
Using a plastic spoon, thoroughly mix the plaster and water.
Knead the modelling clay well until it is soft and pliable.
Coat the sea shell with petroleum jelly and press it firmly into the kneaded clay, so that it leaves an impression in the clay. Petroleum jelly should be enough to prevent the shell from sticking to the clay.
Carefully remove the sea shell so that it leaves behind a mould in the modelling clay.
Fill the mould created in the clay with plaster of Paris.
Smooth the plaster using fingers of your hands or a flat plastic spatula, to form a flat surface.
Place the clay mould containing wet plaster on paper towel or newspaper and set it aside to harden at least overnight. It would be better to leave the plaster for two to three days.
After the plaster of Paris has dried completely, free the fossil by carefully peeling the clay away from it.
Your homemade fossil is ready.