First, here is the theory behind the project:
When light is emitted from an incandescent bulb or an LED (less so for an LED) the light spreads out in multiple directions. The purpose of a reflector is to redirect the wasted light and concentrate it, thus making the output of the flashlight brighter.
For more, you can look here:
Step 1: Parts
Flashlight (can be a penlight or another flashlight)
What you'll use: