The Approach for Requirement Gathering will be either of the two -
Gap Approach
It is the approach where we know that we are fulfilling the need of customers' type of business. Here, we present our ERP customers as it is, and the customer give the changes he wishes to have in each module and screen.
As-Is Approach
As-is Approach is applied when we get to a customer who is in need to build new modules or features which are not built. Mostly it is applied to industries which need different and unique workflow. Here we capture the workflow as it is and build the system accordingly.
In case of modules or features which does not have an existing screen, along with Acceptance criteria, use screen mock ups using tools like Balsamiq. But always highlight that the mock up is only for illustration purpose and not be construed as replica of actual screen in ERP.