Summarize this article:

You'll be working with modeling clay and plaster, so, needless to say, this isn't the cleanest of projects. Lay some newspapers down and let's get started. You'll need:  Plaster of Paris Water Modeling clay Petroleum jelly Small natural objects such as sea shells Paper cups Plastic spoons Newspaper or paper towels Generally, leaves, shells, branches, or bones work best. Just make sure you have enough clay and plaster to cover it.

Summary:
Clean the area. Choose an object for the template of your fossil.