... I'm not by any means the first to attempt a solution to these problems. Earlier this year Giny showed how she uses a laminator to make great reusable templates and a while back Sabine Alienor shared her genius method of creating a very durable template from shrink plastic. Both methods impressed me enormously, and I've got some shrinky plastic to try Sabine's method. But I lack a laminator at home and suffer from an allergy to arithmetic - even the simplest calculation of shrinkage is outside my comfort zone. So, hoping to avoid mental exertion this afternoon if possible, I wondered whether there might be a really quick-and-dirty alternative. So that's what this is - just another alternative, using the materials to hand.
Experimenting with various types of plastic packaging, some of which proved surprisingly hard to cut (and probably weren't polymer safe), I eventually arrived at the idea of using the side of a square (squircular?) plastic milk bottle - the type of food-safe plastic that's also polymer safe. It proved easy to draw on with a marker, and delightfully easy to cut. (Note: As far as I know this type of plastic isn't biodegradable or otherwise liable to disintegrate, but time will tell - and in the meantime any templates or stencils you make with it will be perfectly safe to use with polymer clay.)
These templates (and, if you use a blade or fine scissors this plastic will support some detail, so stencils are also a possibility) are reusable and firm enough to lean your blade against without too much fear of slipping. The plastic adheres well to the clay; this stickiness means it needs to be peeled off carefully, but there are some advantages. It's flexible and you can use it to support thin layers of laminate, peeling off the template once in place. I wonder whether this might even be helpful when insetting shapes into a sheet of clay.
A plastic milk bottle - the sides of a 2.7l bottle gives two decent sized flat pieces, but smaller is OK too.
A largish pair of scissors (kitchen scissors ideal)
A smaller, sharp, cutting tool - a small, pointy pair of scissors or a craft knife or scalpel. Those sharp little nail scissors with curved blades are great for going round curves, and the point helps you get into the shape with minimal damage; if using a craft knife or scalpel, please use a cutting mat!
A Sharpie or other marker