Once you have created print on demand products using these instructions : https://gateway3d.zendesk.com/entries/36448733-G3D-ACP-Creating-Print-On-Demand-Products you will want to add them to your Magento Site
1) Log Into your Magento admin Panel
2) Locate the product you want to turn into a "Print on Demand Product" and edit its attributes
3) Under "Gateway3D Personalise-iT" tab each Magento product has a field called “Print-on-Demand Ref” which should be filled in. No iframe URL should be specified for a “Print-on-Demand” product and no custom options are required.
ALL PRINT ON DEMAND PRODUCTS NEED 2 ATTRIBUTES TO WORK
1) (SUPPLIER) COMPANY REF ID - THIS IS THE REFERENCE NUMBER FOR THE SUPPLIER THAT WILL PRINT THE ITEM WHEN PURCHASED (IF YOU DO NOT KNOW THIS CONTACT THE GATEWAY 3D SUPPORT TEAM WHO WILL BE HAPPY TO HELP AT SUPPORT@GATEWAY3D.COM)
2) PRINT-ON-DEMAND SAMPLE REF - THIS IS THE REFERENCE NUMBER FROM THE GATEWAY 3D ADMIN PANEL YOU RECEIVED AT THE END OF THESE INSTRUCTIONS https://gateway3d.zendesk.com/entries/36448733-G3D-ACP-Creating-Print-On-Demand-Products
Save your product (ensuring it is enabled and in an active category) and it will be available for purchase on your site