Judy covers the traditional GP PO Approval (not the new R2 stuff) in Activating Dynamics GP 2013 Purchase Order Approval
Activating Dynamics GP 2013 Purchase Order Approval – Etelligent Solutions
Sales, Purchasing & InventoryGP 2013
Archived post from 2014. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.