In PSI, there are two ways to generate a purchase order. The first one is to create one in PSI->Purchase; the second is to create an order when adding a contract in CRM.
1. Create an order in PSI->Purchase
2. Create an order CRM
The rest of the workflow is similar to create a sales order.
After creating the PO, you will wait for the warehouse staff to receive the order. Once it is received, click "Finish" to complete the PO process.