Have a pizza while you read the rest of these instructions.
Step 1: Materials and tools.
The polystyrene disc from under a store-bought pizza.
Sharp knife ("X-acto" or similar. The ones I use come under the heading of "similar" - they cost me Â£1.99 ($3-$4) from my local Morrisons Supermarket).
Sticky tape. You may need to use duct tape: if your pizza base is greasy, normal tape sometimes won't stick.
Optional: paper template. I needed to use the template because my pizza base was black, and so is my pen. Some bases are white, and you can draw your pattern directly onto them.
Step 2: The parts.
Cut a square, and draw a line corner-to-corner. Mark a point one quarter of the way along the diagonal. Draw lines to the other two corners and cut along them. You will have a wide kite-shape and a chevron.
Stick the chevron back on the square, but at the opposite corner to the one you cut off. The diagonal line on the chevron should line up with the diagonal line on the larger part. Check the photos to see what I mean.
Draw the shape onto your pizza-disc (most of the flat pizza bases seem to have a pattern of square dimples, which makes life easier - you can count along them and use them for guide-lines if you want to draw your shape directly onto your base) and cut it out with the sharp knife.