Design each star in four steps: process, grain, dimensions, facts

The Kimball Group describes dimensional design as four key decisions, made in order:

1. Select the business process: an operational activity such as taking orders, shipping, or registering a student.
2. Declare the grain: exactly what one fact table row represents.
3. Identify the dimensions: the context, such as who, what, where and when.
4. Identify the facts: the numeric measurements that the process produces.

The order matters. Choosing dimensions before the grain is fixed leads to tables that mix levels of detail. Model one process at a time, and talk to the people who run it. Microsoft's guidance offers a useful listening trick: when someone says they need sales by salesperson, by month and by product category, each "by" names a dimension.