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While plastic ones probably could be used, they're liable to split when cut and be more fiddly, so this article is focusing on the cardboard ones. Any color is fine––if you've been collecting them and have a variety of colors, you might like to use a few different ones to liven up the baskets. Cut as many cups as baskets you'd like to make. Cut with care, to maintain scallops around the top of the cup. Cut circles from the lid of the egg carton. The circle should be the same diameter of the top (open part) of the cup.

Summary:
Use cardboard egg cartons. Cut out a cup from the egg carton base. Cut a base for the cup.