When holding a workshop to collect requirements for a BI project you can take two broad approaches.
When customers know what they want it's easier to think about dimensions and facts i.e. £ sales by customer, product,brand, time. Or £ Operating Expenses by Account, Department, Cost Center, Legal Entity.
No need to mention Dimensions/Facts to business users as this is too technical. Rather talk about about Context and Numbers.
You can use an example Purchase Order form to tease out what will eventually arrive in fact/dimension tables. If the users have the right subject knowledge they probably realise the £ Operating Expenses is not available by Product. This will help you determine how to model the fact tables.
Sometimes users don't quite know what they want in each case you need to delve into processes.
For instance, take a Billing process. It can be helpful to turn this into a process diagram as it will help tease out requirements. It can also be used later to help document functional requirements when used in combination with an ERD.
Don't assume users know what they want. Adapt your action to the situation at hand.