Lots of time staring at photos and lots of math and creativity.
This is a template I created of the small gas gun from Witch Hunter Robin. It is scaled to an average handgun size, though it is sized to scale with the standard gun.
I spent an evening, or about 8 hours, working on the math for the scaling and drawing the completed gun. I used five different pictures for references and had several more in case I needed more references.
I made the template so that my friend
can make the gun as a prop for our Witch Hunter Robin group cosplay.