DogmaModeler is a free, open-source ontology development tool intended to enable non-IT experts to model ontologies.
Protégé is a free, open-source ontology development tool created by the Stanford Center for Biomedical Informatics Research at the Stanford University School of Medicine.
The following steps, adapted from Ontology Development 101: A Guide to Creating Your First Ontology, provide a general guideline for approaching ontology development.
1. Determine the domain and scope of the ontology.
2. Consider reusing existing ontologies.
3. Enumerate important terms.
4. Define the classes & class hierarchy.
5. Define the properties of classes.
6. Define the facets of the slots.
7. Create instances.