Things You'll Need
Instructions
Preparation
Identify a fossil to re-create. A good online resource, rich in illustrations, is FossilMuseum.net. Some good candidates are the archaeaopteryx or juvenile pterosaur (which you can re-create with chicken bones) or a fossil fish.
Boil the animal bones for at least a half hour to remove any marrow and prevent any decay. For a fossil fish, it is easiest to boil the fresh fish in a large pot of water on the stove until the meat falls away; this should leave a complete skeleton. Add a cup of vinegar to the water to soften the bones and ensure they are completely clean. Allow the bones to dry.
Arrange the dried bones into your dinosaur shape on a flat surface if you are using chicken bones. Use the photos on FossilMuseum.net as your guide to arranging the bones. You will later press the bones into plaster in this same shape.
Create the Fossil
Dig a shallow hole in some dirt, in a rough-square shape, with a garden trowel or shovel. You will use this as a mold in which to pour your plaster. Fossils are typically excavated in rough shapes, versus perfect circles and squares.
Mix plaster of paris per the manufacturer's instructions (typically a 1-to-1 mix of plaster and water), and pour it into your dirt hole. Spread the plaster smoothly with a trowel or spatula.
Press the bones into the still-wet plaster in the arrangement you created earlier. You can just use your hands to do this.
Dust the still-wet plaster with common sand, pressing the sand into the plaster. This will simulate sandstone.
Allow the plaster to dry overnight or, if your dirt is in a portable, oven-safe container, bake it in the oven per the manufacturer's instructions.