Our method front-loads the decisions that are expensive to reverse: chart of accounts, costing method, warehouse and route design, product structure, approval rules. These are made in design workshops with the people who will live with them, documented, and signed off before build starts.
Build happens in short cycles. Every two weeks we demonstrate working functionality on your own data, so misunderstandings surface while they are cheap to fix. Data migration runs as a parallel workstream from week one, with scripted loads and at least two full mock migrations reconciled against the old system.
Users write the acceptance test scenarios, because they know which edge cases actually happen. Training is delivered per role, on the transactions each person performs, in a test environment that mirrors production. Cutover follows a rehearsed runbook with a formal go/no-go decision, and our team stays for hypercare through the first period close.