Most boys at a young age learn or figure out how to make a ninja-like hood from a black t-shirt by tying the sleeves behind the head with the neck hole placed over the eyes. Modern ninjas, such as SWAT teams and black ops, wear a custom tailored black balaclava, which is doubly cool since you can wear it just on the top of your head like a stocking cap, and then when it's time to go into action, you pull it down over your face and cock your mp-5.
But real ninjas lived and flipped out and killed people long before the t-shirt was invented in 1913. Having no t-shirts, they used a large square of black silk, about five or six feet square. A silk blanket such as this has many uses for a vagabond; you can wrap up all your personal effects and tie it to a stick, lay it out and have a picnic, etc. With your own black silk blanket, you too can make an authentic ninja mask in the traditional fashion!
First, fold the square of fabric diagonally, corner to corner, making a right triangle. Fold this in half, from one 45 degree corner to the other, making another, smaller right triangle.
Taking the two 45 degree corners in your hands, hold the triangle over your head and move your head inside the fold next to the right angle. Look up, so you can see out between the two sides of the fold. Bring the ends down tight against your face, so that only your eyes are showing. Tie the corners in the back. Now you're ready to jump out of a tree and ambush someone.