You make a new layer. Make it cloudy (however the heck you choose to do this) with dark and light spots. Make it COMPLETELY opaque (you can't see anything through it).
Then set it to... oh, I don't know, say 10% and set it to "Screen" in the layer options.
He means make a cloudy layer, then play with the layer options like how much of it shows (% of opacity) and in what method it's applied (normal, multiply, screen, luminosity, etc...)