In order to build a "working" pair of custom sunglasses, I used the lenses from an existing pair of my older sunglasses. You can also just buy a pair for the lenses for a few dollars at the corner store.
I took my old pair of sunglasses to Techshop in San Francisco and scanned them on their Next Engine 3D scanner. First I sawed the plastic frames in two parts using their bandsaw at the center line between the lenses. This way, I'd only have to scan half of the sunglasses. When I built them in CAD using AutoDesk's Alias Studio CAD software, I easily mirrored them over centerline. I also twisted the folding part (arm) of the frame apart at the hinges so I could scan both half of the frame and one arm together on the circular scanning platform.
I built a custom frame shape in Alias Studio software around the existing lenses. In Alias I was able to import an image plane into the software (or your own sketch) to get an idea of what I wanted the finished product to look like.
I had it printed in five parts. You see, I printed two arms, and them the front side of the frames. For the back side of the frames, I printed two parts that would sandwich the frames to the front part. I also used the metal hinge parts from the old pair of sunglasses and incorporated them into the new pair.
It took some sanding and epoxy work to fit the three parts together, but I think it was worth it! I painted them frames after I masked the lenses out too.
Have fun and be creative!