I made mine at http://techshop.ws
Step 1: Collect some cardboard
You're going to need a cardboard sheet uninterrupted by cuts or flaps. The sheet should be at least the height of your trash can minus one inch, and its width should be the outer circumference of the trash can plus several inches. You'll be cutting the box into two pieces and gluing them back together, so you need use the extra width as overlapping surfaces to glue together. I had maybe 16 inches extra and thought that worked out pretty well.
The corrugations should run vertically top to bottom because that will help us bend the cardboard to the curves and/or corners of the sides of the trash can, and will give some much-needed strength to the pull tabs of the insert.