Here is a diagram showing the rough concept. the black line represents the pattern you are cutting around with the black text on the inside of the design..
the green is the first cut made with a punch or curved chisel (
spindle roughing gouge) not what it's designed for but I imagine you can find similar tools that would perform a similar job. leatherwork and woodwork tools are very sharp and can be found in many different profiles.
you then cut up to that initial cut with scissors or a sharp blade, however you normally cut. the advantage is the corner is already done so you can cut straight lines right up to your pre-existing corner cut. The caveat is I have no idea how the material you're using will respond to being punched through in this manner. It should be OK on a solid cutting mat, I'm gueesing:o
