Once you have setup your templates in your Order Printer app, you can print documents from within the Shopify admin in 2 ways:
Printing a single order
In your Shopify admin
Go to the Orders section
Click on an order to open it
In the "More actions" dropdown, select Print with Order Printer or Print with Order Printer Pro
Printing multiple orders
Shopify allows you to select up to 50 orders, and then open apps with those orders selected and ready for printing.
In your Shopify Admin
Go to the Orders section
Put a checkmark on the orders you want to print for
In the pop-up actions bar, click on the three dots, and select Print with Order Printer or Print with Order Printer Pro