Map the current material flow first.
Follow purchasing, receipt, storage, issue or production use, finished output, sales and finance. Identify where quantities, values or responsibilities become unclear.
Separate standard manufacturing configuration from real gaps.
Routes, bills of material, work centres, inventory rules and accounting configuration should be reviewed before custom development is proposed. A custom module should solve a documented operating gap, not simply reproduce an old screen.
Reconcile stock and cost before go-live.
Opening quantities, warehouses, units, valuation assumptions and financial balances need a common cut-off. Production testing is not complete if the operational flow works but stock value or accounting does not.
Test the exceptions.
Partial receipts, scrap, substitutions, delayed production, rework, returns and landed-cost effects often reveal more than the ideal demonstration path.