In this project, I took a pair of Oakley Half X glasses and cut the lenses so that they no longer touched my cheekbones.
Yes, these are $300+ sunglasses and I took a belt sander to them.
Step 1: See Where They Glasses Are Hitting.
Put the frames on and note where the lenses are touching your face. It's a judgement call as to whether it is feasable to remove enough material to make the lens fit, how much to remove, and whether the remaining shape will be aesthetically pleasing.
Step 2: Mask the Lenses
On the other hand, it worked well enough.
Your main goal in masking the lens is to delineate exactly how much lens you will be grinding off. Put the tape on the part of the lens you will be KEEPING and leave the part you will be grinding unmasked. This way you can measure/eyeball the masking on both sides to make sure that you have masked symmetrically.
In this case, I'm removing just enough material to eliminate the "Polarized" etching on the corner of the lens, then blending the corners back in to match.
Step 3: Grind
You can either freehand it using a dremel-type tool and an abrasive drum (the kind that comes in almost every dremel kit), or do what I did and take it to a beltsander with a relatively fine grit belt. I chose the belt sander to ensure that the bottom edge had a nice straight line to it.
Step 4: Keep Grinding
If you build up a burr because you went too quickly, lightly grind on a slight angle towards the burr to cut it off, then continue. Alternately, you can stop grinding and roll the burr back over the edge with your fingernail if it isn't too thick yet.
Step 5: Test Fit
If you find that you need to remove more material, go back to the beginning and re-mask the lenses.
Step 6: Finish the Edges
During this step, I removed the masking tape. Yes, there was more risk to the lenses, but I found it easier to make sure I was removing the burr from the lens edge without the tape there. Surprisingly, I found that the dust from grinding the lens did not scratch the coating at all, which makes sense since the surface coating of the lenses is much harder than the actual lens itself.
In this picture, you can see how nicely the edge comes out. There is no damage to the mirror coating, and if you take your time, it will look like a factory edge.
Step 7: Finished Product
It was actually much much much easier than I expected. Practice on a pair of dime store glasses first until you get the hang of working with polycarbonate.