start with whatever image you are useing...
use the magic wand tool to select the sky
duplicate the layer and cut out the clouds
place your cloud picture below that layer and adjust it as necessary
then adjust the coloring of both layers till they match up (keep in mind how shadows work when working with sky's)
and finally... to make the before and after pic
merge the new layers, and then use the eraser tool with a large brush head and make a large diagonal stroke across half of the top layer
fun stuff
edit: did anyone ever take the time to download those?
I think the link will die after 30 days if there is no activity