A standard size dog food can or a soup can are perfect.  Remove the paper label, the can lid, and wash it clean with warm water.  Dry before proceeding to the next step. They need to be long enough to wind around the can.  Try not to touch the sticky ends, or get them dirty, otherwise the trap will not work. Press the duck tape tightly against the can with your hands.  Give the duck tape a gentle rub in order to transfer the stickiness. The surface of the can will now be sticky.  Gently touch the can to see how sticky it is.  If it is not very sticky, repeat with new lengths of duct tape. Put the lid over the base of the flashlight.  This will create a base for the flytrap.  It is best if you can find a UV flashlight, as flies are mostly attracted to UV light. Sit the can upright so that all the stickiness is available to catch flies.  Switch on the flashlight and place it into the can.  Make sure that the flashlight is upright, and has new batteries in it. They'll be attracted to the light but get stuck on the sticky sides of the can. If you have successfully caught flies with the can, it is best to throw it away.  Make sure to use a pair of gloves when you contact the can, so you do not have to touch the flies.  It might be best to have a plastic grocery store bag ready to place the can in, before you throw it away in the trash.

TL;DR in Chinese:
寻找合适的罐子。 剪下长长的布基胶带备用。 用胶带绕着罐头。 把绕着罐头的胶带撕下来。 用胶带将一个小小的手电筒贴在罐头盖子底下。 晚上的时候，把罐子放到屋外。 等待苍蝇到来。 更换罐子。