You will need two pieces: both pieces should  be 6-inches by 3-feet. Alternatively, you can purchase one piece of cloth. This type of ninja mask is less realistic but much simpler to make. Simply lay the piece of cloth on a flat surface and cut an oval out of where you would like your eyes to be. Once cut, place the cloth against your face so that your eyes and upper nose are the only parts of your face showing, and tie the ends of the cloth behind your head. Hold both ends of the piece and place it on your mouth before drawing both ends to the back of your head (as if you were putting a bandana on). Cris-cross them and then wrap them along the back of your head and around your neck (make sure this is not too tight!). Tie the two ends into a knot at the back of your head. Holding both ends, wrap the ends under your chin, and then pull the ends back towards the back of your head. Tie them at the back of your head.

TL;DR in Chinese:
自己剪裁布料，或者拜托布料店店员帮你剪好。 用第一块布（布条A）包住嘴和鼻子的下半部分。 将布条B蒙在头顶上，把布条两端拉到下巴底下，再绕到脑后打结。