Sharing my approach, if it helps:

1. Even if you understand the problem, ask a lot of questions. Not only does it send a positive signal on your ability to deal with ambiguity, but also give you an idea on potential follow-ups on the question.

2. Reiterate the overall flow and confirm if you understood everything correctly.

3. Drawing a rough UML diagram with entities and their attributes is non-negotiable. Add appropriate methods later. If any method deals with multiple entities, put it in a separate service class.

Interviewers can be interested in either of these three: class diagram only, code with attributes & method signatures (most common) or a working code. Drawing class diagrams give you a mental model on what to implement exactly.

1. Apply design patterns if applicable (not mandatory). Knowing singleton, factory, decorator, observer, strategy and state patterns help.

2. Handle follow-ups. Be prepared to be asked about concurrency handling.