Precision Cutting Volan Inlays?

Hey all,

Any advise on how to precision cut a deck inlay?  I was able to do a perfect bottom tail inlay, but it took me x2.

I see these super shapers on YouTube just-frikkin-slicing with the scissors, I don't have that confidence.  Is there a fail-proof tricks on cutting these inlays without biffing it and wasting glass?  I'm just doing a simple daimond.

Please and thank you.