Now that you have a potato, you might be wondering just how we will be making plastic out of this? The answer: extract the starch. How will we do this? BLENDER! Since potatoes are about 95% starch (besides the water), making a potato into a powdered starch form, is relatively easy.
A note to younger viewers: be sure to ask your parents if it is alright to use a peeler, knife and blender, and always use caution when working with exposed blades.
1) Get a potato, and wash it.
2) Use a peeler to take all the skin off.
3) Cut the naked potato up into cubes your blender can handle.
4) Add about 1 cup of water and the cubes to the blender, and turn it on high for a minute or two.
5) Use a coffee filter to strain off the cloudy water.
6) If you plan on making the plastic right away, drying the mixture is not completely necessary, but if you plan on storing it for a while, spread it out on wax paper in a sunny area for it to dry (it could get moldy otherwise).

Heres a suggestion by legionlabs to purify the starch extracted from the potato, by removing unwanted cellulose:
"Given only potatoes, you might be able to increase the purity of the starch for your process by removing cellulose using this reagent, which can be made from commonly available materials:

http://en.wikipedia.org/wiki/Tetraaminecopper_hydroxide

You can reuse the reagent by precipitating the cellulose out by acidifying the mixture, filtering out the cellulose, and making the reagent basic again."

If you do not wish to make the starch from scratch, you can order pre-made higher quality starch online, or buy it at your local grocery store. Corn starch and tapioca starch also work well to make plastic with. On to step 3...

Now that you have gathered all the necessary materials and ingredients, its time to mix up a batch of potato plastic. A note to younger viewers: be sure to have a parent with you when using the stove, I don't want you to burn down your house because of this instructable! Also, beware of the starch plastic resin when you are pouring it into a mold, it is very hot and will burn you if it gets on your skin, you can never be too cautious with boiling hot substances.

1) Measure out 60 ml (4 tablespoons) of cold water and pour it into the beaker or container your heating the mixture in.
2) Measure out 10 grams (or about 1 tablespoon) of starch ( the potato / corn starch you made or bought) and add that to the water.
3) Add 5 ml or about 1 teaspoon of acid (vinegar) to the mixture.
4) Add 5 ml or about 1 teaspoon of glycerin to the mixture.
5) If a colored plastic is desired, add in the food coloring now. About 5 drops is good enough.
6) Turn the burner on low and constantly stir the mixture. When it starts to thicken up turn the heat up to medium and stir even more. When it starts to boil, keep boiling it for 5 minutes. You want it to be very clear and sticky (not like toothpaste though, think flubber)
7) You should now have a "gooey" substance that you can pour into a mold, or you can pour it onto a sheet of aluminum foil/silicone heat pad to dry.
8) Depending on humidity, it should take about 1 day to dry in a sunny place. You can dry it faster by putting it in an oven set to 150 F for 1-2 hours.