Identify the domain
Decide what the graph is about and, more usefully, what it is not about. For a retailer that might be products, customers and orders, and the relationships between them. Scope set badly here is the most expensive mistake available.
Define the entities
Name the things you want to hold information about. These become the classes in your model: Product, Customer, Order, Store. This is the vocabulary the rest of the organisation will end up using, so it is worth arguing about.
Define the relationships
Say how the entities connect. Purchased, viewed, recommended, supplied by. These are the edges, and they are what makes a graph worth having, because they are the part a relational schema makes expensive to traverse.
Determine the attributes
Decide what you record about each entity and each relationship. Name, description, price and category on a product; the date and time on a purchase. Attributes on relationships are the ones most often forgotten and most often needed.
Model the graph
Turn the decisions above into a schema: an ontology for a semantic graph, or a property graph schema. This is the blueprint, and the thing every later step is checked against. In Studio it is drawn rather than written, and the formal artefact is generated from it.
Map the data
Connect real sources to the model: which column is that property, which table is that class, which nested field is that relationship. Historically the longest step by a wide margin, because it meant writing an application per source. Here it is configuration against the model.
Validate the model
Check that what came out represents the domain and answers the questions it was built for. Real output, in a real database, queried the way the application will query it. A model validated on a diagram has not been validated.
Refine, and go again
Change the schema, the entities, the relationships or the attributes in light of what validation told you, and run the lifecycle again. How cheap this step is determines how good the finished model gets.