To create a Purchase Order, go to the Projects area and open a project, then click the Purchasing link on the left nav bar.
Select the items you would like to create a PO for, then click "Order Selected Items".
Select the shipping address, along with your preferred shipping method (you can set this up in the Settings area). Add any additional notes, and click Save.
All of those items will now list the date ordered. Click the name of the PO to view it, or hover your mouse over the name of the PO and click the down arrow to download it.
Once you have received the equipment from the vendor, you can go back to the Purchasing page to mark the items as 'Received'.
If you need to adjust the quantities of the items on your PO, or add some items to the PO that weren't in the project, click here.
Here is a tutorial video on Purchase Orders