This will separate the sides, keeping the paint from seeping through. If you don't have cardboard, use some other sturdy material that you're not worried about getting paint on.  Lay the shirt flat and smooth out all the creases. The cardboard insert needs to be the just short of the width of the shirt when laid out. If you're using a t-shirt, consider wearing a sweatshirt underneath that is your turtle's color. For ideas or a template, go online and see artists' depictions of the Turtles or other costume designs. The yellow is the bottom of the shell -- make it fitting to your size. It's easiest to use a paper plate as your palette. You can just throw it/them away afterward and not worry about the mess.

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Write a summary of the previous text in Chinese:
在T恤里塞入纸板。 在T恤中央画一个大大的黄色矩形。