When building this device, the first step is to program what you want the LED and the photoresistor to do in Arduino programming software.
building a photo resistor is very easy if you follow this guide step by step.
Step 1: The Code
Use the code found here Instructables does not paste code very well. You will find a comment next to each line of code.
Step 2: Circuit Design
Step 3: Using Serial Port to Find Value
In Arduino IDE press CTRL+SHIFT+M to open the serial monitor. If everything was wired correctly you will see values that show the light the photo resistor is detecting. Place your hand over the photo resistor to find the new value of when the photo resistor is covered. With this photo resistor in changed from 850 to 650, so set Pr_Input = 0.