Step 1: The Mask
First You take a black T-shirt and put it inside out after that the T-shirt On with the tag on your back bring the front side of the collar up above your nose and ears then the back of your shirt that would lineup with your waist pull it over your head and Align it so there is a little slit for your eyes. The T-shirt sleeves should be sticking out, one on your left and one on you right. Then told them around The back of your head then tie them in a slip knot or just a normal knot... It should look like so.
Step 2: The Body
Start By taking a black jacket or a long sleeve shirt (must have hood) and black jeans/sweats preferably skinny. Put them on like normal. Then pull the hood over your Mask. Add black shoes. Maybe even some sort of ninja headband Or ninja Star (Optional). As you see this is a very easy fun and good lookin costume!! Remember stay creative