Step 1: Round Up Some LEDs!
First you'll need to get however many LEDs you want. You'll then want to connect the LEDs in a series, leaving a positive and negative wire.
NOTE: This works with any LEDs, but I have found that it works best with Red LEDs
Step 2: Go Towards the Light!
Then you'll want to get a light source (lamp or daylight) and put the circuit right in the strongest part of the light.
Step 3: It's As Simple As That!
Just hook up the wires to a voltmeter and you'll see how much power they are generating. I was able to produce 1 volt just by using 3 LEDs and a bright lamp.
(As if you couldn't tell, this is my first ible, so some advice would be greatly appreciated)
Inspiration for this project and more information on how this works can be found at https://www.instructables.com/id/Better-LED-as-light-sensor