When you're ready to send your PO to your supplier, open the order and click Email/print. The order will be shown using the template you have set up for Purchase Orders based on the channel branding for the order's channel.
Here's a typical PO template:
Click Email and choose your recipient(s) email address.
Enter a message for the email, and click Send.
The Purchase order will be updated to the status you have set for this workflow, and a note will be added to the order history. A PDF copy will be stored for future reference.
If your supplier uses different SKUs for your products, then you can create a cost price list specifically for this supplier, containing their SKUs.
Grid mode for purchase orders
If you're ordering products that have lots of variations, you might prefer to use a template design that puts one of the options into a column. This is the same purchase order as the above template, using 'grid layout' :