Here's what you'll need:
1. Fence wire (or chicken wire) with square holes.
You can find this kind of fence wire at most any hardware store. It's sold cheap and by the foot. The size of the square holes range anywhere from 1/16" up to 1/2", allowing you to make many different QR code stencils in many different sizes. In this instructable I'm using 1/4" fencing.
2. Masking tape (the wider the better)
3. Xacto knife (#11 blades recommended)
3. Pliers for cutting wire (the smaller the better)
4. Two sharpies of different colors.
5. Black spray paint