Tutorial: Getting Started with Protege-Frames
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Where do I start?
You might start by determining what you are going to use the ontology for, and how detailed or general the ontology is going to be. Among several viable alternatives, you will want to determine which one would work better for the projected task, be more intuitive, more extensible, and more maintainable. Remember that an ontology is a model of a real domain in the world and the concepts in the ontology must reflect this reality.

After you define an initial version of the ontology, you can evaluate and debug it by using it in applications or problem-solving methods or by discussing it with experts in the field. As a result, you will almost certainly need to revise the initial ontology. This process of iterative design will likely continue through the entire lifecycle of the ontology.