[DR - Robin, this shouldn't preclude you from using masks. However, if you'd like to set using Remove Color Matte or Set Matte or any other keying effect, I'd say go for it. If you're still stuck, tell me specifically where in the tut you're getting stuck and post here and I will try to give you some additional information.

I always try to keep the tutorials easy, but I freely admit that I can take certain things for granted that others may not know, so please let me know.]

I recreated this and used less opposing angles for the directions of the separate layers, it gave a more flowing look to the fog rather than it being created literally out of thin air

Check out Andrew Kramer’s clouds in the earth zoom tutorial for using fractal noise inside AE.

Keep it up!

[DR - Thanks for the kind words. You are absolutely correct, you can create some of the same kind of stuff with Fractal noise in After Effects. which one you use depends on what you're going for and what you're comfortable with. Thanks for the comment.]