Here is how.
Find any #6 plastics, some examples are fresh berry containers or deli lunch containers.
Cut out the flat bottoms and tops of the containers.
Rough up the surface with a piece of sandpaper, or you could just rub it on the concrete for a minute. Your just prepping the surface so it will hold the medium of your choice.
Apply medium of your choice. Markers, colored pencils, rubber stamping all works great. Just keep in mind you color will come out super concentrated as the plastic shrinks, so plan accordingly.
We used a toaster oven which heats up faster than a conventional oven and saves on energy.
There is no specific time for baking the plastic, you have to sit there and watch it, and it goes pretty fast.
Place your plastic on a piece of baking parchment on a pan and make sure your oven is pre-heated. Your shrinky will curl and shrivel and your heart will jump because you think it's ruined! But fear not. 99% of the time it flattens itself out. When you take it out of the oven slide the parchment paper off the tray onto a hard flat surface like a counter, and put another piece of parchment on top. Have a plate, or something hard and flat, and press down on your covered shrinky to flatten it out even more.
You have to be quick when it comes out of the oven because it cools quickly. You can always put the shrinky back in the oven if you are not satisfied with it's shape.