Start off by choosing a landscape that you would like to animated your clouds infront of. Past your image in a new canvas.

Create a new file with the same height as your background image, but with twice the width. Make sure your foreground and background colors are black/white, and choose Filter > Render > Clouds

 

Copy and past your rended clouds back into the cnavas with your background image. Change the layer mode of your cloud layer to Screen.

Open up your animation window by choosing Window > Animation

Create a new frame by clicking on the arrow to the right of your animation window, and choosing "New Frame."

In your new frame, drag your clouds to the left, so that the right end of your clouds image is in line with the right end of your canvas.

Tween your frames by clicking on the arrow to the right of your animation window, and choosing "Tween..."

Depending on how fast you want your clouds, input a # of frames. The larger the #, the slow the cloud with move, and the larger you file size will be.

Save your image as a .gif, by choosing File > Save as Optimized.