I will show you how to make a ninja mask out of a t-shirt. At the end, it is really cool, but don't walk around carrying a gun in public with the mask on, You look like a terrorist.
P.S. I didn't make this up myself, I just didn't find an instructable about this so I decided to publish it.

Step 1: Getting the T-shirt

To start off the mask, first decide what color you want your mask to be. If you want red, get a red t-shirt, black, black t-shirt...etc. after that, turn the shirt inside out.

Step 2: Putting It All Together

First, Hold the t-shirt with the tag at the back. Then pull the t-shirt over your face so that you can see through the collar, but don't put your head totally through the collar. Next, using the sleeves, pull it tight on your face and tie a knot in the back. Make sure the knot is tight. After that, pull down the collar so you can only see a small part of your forehead, and stick the tag in one of the folds. then, take the bottom part of the collar and pull it up over your nose. Take the rest of bottom of the shirt and gently pull it down so it isn't scrunched up.