The easiest way is not to make them but to grab them from sattelite images. Grab a picture of clouds and use the 'select by colour' tool in photoshop, adjust fuzziness and then invert the selection and delete.
Like this (the first one is a png with a transparent background and the second one is a jpeg with a background).
To make gradients from snow to grass etc I just use a really big soft edged brush to do it. Make sure you have your land selected so you can paint inside the land areas and use the edge of a big soft brush. Hope that helps!
If you want to use rendered clouds, it is worth warping them a bit with liquid warp/iwarp then differnece clouding them a fiew times and playing with levels.
Lastly, duplicate the clouds and put a dark grey, blurred copy beneith the white to give it more depth.
here was a quicky:
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