Map the domain that your product will be part of, so you know the key concepts and the relations between them.
Talk to stakeholders in the field, and iteratively create a conceptual model of the domain in which the concepts and relations are as clear (or formal) as possible. Make sure to look at the domain from different angles.
- One, or preferably multiple, knowledgeable stakeholders.
- A technique for capturing the domain in a model, possibly the UML class diagram.
- Sufficient time to go through several iterations.
One of the first steps in an analysis is often the creation of a domain model: once you know you understand the domain, you can perform subsequent steps.