So with Redbubble and Society6 it's very easy. You just upload a jpg or a png that is in CMYK, 300dpi, and it will print fine because they have nice offset printers (it will also print as a square I think unless there's a way to indicate otherwise) When your customers go to order, they will choose the size and pay the shipping, and the POD service does the rest. You get a smaller portion of the profit (again it's like 20 cents) and you only get paid if you reach like 20 bucks total, so make a lot of it. It's a nice little passive income.
You do have to indicate "sticker" and make sure it's enabled--it's under the same category as greeting cards for some reason. You'll see it when you decide to do it.
As for printing your own stickers, I don't know.
