In the Point of Sale click the hamburger menu on the top left-hand side of the screen. Click the "Settings" option.
Select Printers, and select the "Create Printer Configuration" button
Search for the printer by clicking "Search for Printers" button. Select the printer you'd like to use and click the "Next" button (bottom right of popup.)
Select the items you would like to print to that specific printer and click "Save" (bottom right of popup.) And now your printer is set-up. It should show a green circle with the word Connected in green.
To learn more about printer troubleshooting, please see Printer Troubleshooting / Repair