First, make sure that your store is fully set up. Then, download the app from the App Store (search for "Bonfire Point of Sale"), and follow the setup steps.
To make a sale:
Open the POS app
Select products from your store
Add items to cart
Accept payment from a customer (tap-to-pay or card reader)
If the cart has a print on demand item in it, you'll be prompted to provide customer information (email or phone number). A link will be sent to the customer so that they can provide shipping information. Bonfire will ship the item to that address.