There are many different approaches depending on your current infrastructure, types of products and levels of customization. The web interface is only the tip of the iceberg, as you'll need to control your production workflow, fulfillment and supply chain (especially if you are doing anything more than a "hobby" volume).

I've worked with a number of companies who have attempted to build their own solution and, unless you do not find a very compelling reason to, I would recommend not doing that :-)

Disclaimer: we have our own web-to-print solution and production management system that I can tell you about, but I know the market quite well and would be happy to learn about your specific infrastructure, type of print products and production capabilities to point you in the right direction. For instance, there are plugins for e-commerce platforms that are simple, cheap and could get you to market quickly. Or you can hire an agency to make sure all of your brand objectives are represented by your print platform. Or any number of choices in between. Feel free to schedule a call to discuss. Happy to help.