I've gotten a few comments on this, so I guess I should clarify that the ring assembles to a tight fit through elastic deformation of the printed substrate.
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Step 1: Crate a "blank ring"
Create two concentric circles of different size (an annulus), then extrude them.
The inner circle should have the same circumference as the target finger. This website should help convert from ring size to circumference: http://lovejoyjewelry.com/ringsize.htm
The length for the extrusion is up to you. I chose .375 inches, just because that's how big another ring I have is.