…is acquiring the Domain Knowledge.
The ontology itself is just a complication.
Knowing Description Logics, memorizing the standard set of relations modeled in OWL 2.0, having custom keyboard mappings for Protégé, top-down or bottom up approaches, Basic Formal Ontology. None of that is easy, and yet it is most likely going to be the easy part.
The hard part of building ontologies is understanding the domain you are building them for. It is the inquisitive, introspective process that you undergo while questioning every single word choice you make for every single piece of data or metadata you are modeling. How will it be put to use. Why.
Next time you see your product manager, your domain expert, buy them a beer and listen.
This knowledge you embed into an abstract machine.