Traditionally, before rotary cutters and such, we made a template of the finished size of the patch (whether square, triangle, hexagon, etc.) Trace around the template, then cut out. It didn't matter if you cut an exact 1/4" outside the line, as the size of the finished patch was what mattered, not the size of the seam allowance. This was back when I only had scissors to cut with. Oh my gosh, this makes me sound so old!!
I used to make my templates out of old greeting cards. But they will start to change size after being traced around many times. So now I would use template plastic instead. Or you could use the top of a cottage cheese container I suppose.
Andi