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Your t-shirt may get stretched out in the mask-making process but you should be able to wear it again. Your arms should not go into the t-shirt. Slide your t-shirt neckline down so that the neckline rests above your eyebrows and on the bridge of your nose. This will give your mask a more streamlined look. Folding the collar also covers the shirt tag. Tie them tightly so that the knot does not come loose later. If you are planning on wearing a full ninja costume, tuck the rest of the t-shirt into the shirt of your ninja costume.
Take a black or dark colored T-shirt and turn it inside out. Slip the t-shirt over your head but do not pull it past your shoulders. Fold both the top and bottom collar in so that the seams are not showing. Take the sleeves and tie them behind your head. Spread the rest of the t-shirt over your shoulders.