So it's called retr0bright
, you can find all the info you need on that website.
Basically hydrogen peroxide + sunlight (or an uv lamp) will unyellow plastic. The reaction is pretty slow however, but you can boost it by adding a pinch of "oxy" laundry booster. Hydrogen peroxide being a liquid, the mixture isn't very convenient to use, so you turn it into a gel by adding any gelling agent (they use xanthan gum, I used agar-agar which is what I could find, but you have to heat it).
Then the glycerin is so that it won't dry too quickly.
Overall it's pretty easy to do. You just have to find all the ingredients, and wait for sunlight.