Make a ninja mask with just any ol' t-shirt (although I would recomment a black t-shirt for proper ninja effect--a tie dye mask would look a bit odd).
Update: I wrote a lengthy diatribe on my soapbox here awhile back on the false appropriation of nerdom--a kingdom that I've been a loyal subject for as long as I remember--by those who've never actually been a nerd.
Only a true nerd would 1) surf the appropriate websites that unearth gems like the ninja link above, and 2) feel compelled to actually see if it works. The result? 100 percent success...100 percent success that these photos will eventually come back to haunt me in the future. It's alright. How many people when they eventually meet their end can say "Yea, I lived life: I once turned a t-shirt into a ninja mask?"
I'm surprised the camera could even capture me, er this ninja in this photo because...ninjas are super fast, secretive and also, generally avoid the flash of a camera.
Less well known are the hipster ninjas.