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

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Copy and past your rended clouds back into the cnavas with your background image. Change the layer mode of your cloud layer to Screen.
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Open up your animation window by choosing Window > Animation
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Create a new frame by clicking on the arrow to the right of your animation window, and choosing "New Frame."
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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.
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Tween your frames by clicking on the arrow to the right of your animation window, and choosing "Tween..."
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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.
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Save your image as a .gif, by choosing File > Save as Optimized.
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