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:
Step 2: Mark Your Cuts
Step one of this project is to mark a spot right in the middle of the bottom of your cup. Make sure the mark is the size of your light (in diameter) and centered. Make the mark on the backside of the cup (so it is visible even after you have coated the interior with foil).