Andy Sather covers Microsoft Dynamics 2013 R2 – Purchase Order Processing/Encumbrance
Microsoft Dynamics 2013 R2 – Purchase Order Processing/Encumbrance – Dynamics GP Support and Services Blog
Sales, Purchasing & InventoryMicrosoft & the GP Ecosystem
Archived post from 2014. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.