Part 1) I'm surprised you haven't heard of print on demand? No matter I explained. Print on demand,like the name suggestion. Is are that made depending on how many people brought. That assuming if you didn't know. But if you do know what print on demand or P.O.D for short Is. But asking how to do it? It depends on your goals. If you want to just try it out. Then I suggest using red bubble.
Not only Is it one of the easy Print on demand sites. But unlike p.o.d like printiful,printiful,etc... It also a marketplace where you can sell products there. And again unlike printiful,printiful,etc... It free. If you made a shirt on printiful that you connect to eBay for example. If the shirt cost $10 on printiful. And you sell it on eBay. for $25. You give printiful it $10 and keep the rest. Where Redbubble does the same somewhat. But instead of paying. They just give you the rest. And the products that Redbubble has. Is clothing,stickers,backs and phone cases
All you need to do is take your renders and you can put them on all the products. If you want to go full speed ahead. Their are other products that you can use your renders on that you won't find on Redbubble or main p.o.d sites.
I would love to show examples but not sure if it will get deleted.