A mask is the one essential piece of equipment for the ninja. Shurikens, swords, caltrops; these are all superfluous, as ninjas are total badasses in empty-handed combat. However, when a ninja is being a badass (all the time), he has to remain anonymous. Mysterious. Delicious. Hence, the mask.
Ergo, if you want to be a ninja, or at least look like one, you need a mask. Preferably black. Let's get to the skinny.
First, find a black t-shirt. If it has something on it, turn it inside out. Then...
    1. Put the shirt on your head backwards, with the tag under your chin (or on it, as it may be) with just your face visible through the neck. THIS IS IMPORTANT: Attempting to follow the subsequent steps in any other configuration will result in serious injury or quite possibly death.
    2. Pull the shirt snug against your face and tie the two arms together behind your head.
    3. Fold the upper cloth under itself (to hide the collar) and pull it down just above your eyes.
    4. Do the same thing with the bottom cloth, and pull it up to just under your eyes.
    5. Fine tune the mask. Elimitate as many creases as possible, and make the opening only as large as your eyes. If necessary, start over, only tieing the shirt tighter, or using a shirt with a smaller neck.
Congratulations. You are a ninja; don't let anyone tell you otherwise. If they do, pop a boner, flip out, and kill them. Actually, you should do that anyway, because that's what ninjas do.
Enjoy yourself. This is utmost in importance.

My own version of the method described in the tutorial of Vern Reld, who seems to enjoy Christmas more than ninjas.