Photoshop :: CS5 - How To Make Shapes In The Sky Out Of Clouds

I have intermediate level knowledge of Photoshop (CS5). I did a one-day course a couple of months ago, where the instructor told us how to put shapes/images/text in the sky and then make it so that it looked like cloud.

For example we wrote something like, "High in the Sky", then managed to make it look like it was like clouds in the sky that were spelling it out. We also dropped a picture of a plane or something then did the same thing, so there was a cloud shaped plane in the sky. The problem is I've completely forgotten how to do this but it so happens I need it for a work project I am doing.