There will be a situation were you will need to create a back-to-back purchase order from a sales order. These are the steps to walk you through. 

Step-by-step guide

1. Click on the menu arrow, and click on Sales

2. Locate your sales order. There is a button at the bottom of the sales order called CREATE PO.

3. You will see that it automatically opens up the PO screen automatically.

If you need to go through how to create the PO, go this link