Start with a business process, not a feature list
Trace one meaningful workflow, such as purchase request to vendor payment. Identify who initiates it, which approvals are necessary and where information is entered twice. This reveals the boundaries an ERP system needs to support.
Name the owner of each record
Decide which system owns suppliers, customers, items and financial records. Integrations become easier to reason about when ownership is explicit and reconciliation is part of the design.
Separate essential scope from later improvements
Choose a first phase that can be tested end to end. Define acceptance scenarios with operational teams, including exceptions, cancellations and failed integrations.
Rehearse migration and adoption
Trial imports expose data quality problems before launch. Reconcile results, train the people who will use the system and agree a cutover plan with an explicit rollback decision point.