OK I'll give an attempt:
Even for me it's mostly not as easy as it maybe looks
Before / while painting, I try to imagine the clouds in three dimensions, how the light falls onto which part of the cloud and where the shadows are. There are many different cloud types and every type has distinctive shapes and structures.These different cloud types are also at different altitudes. You should be aware of, which type of cloud you want to paint and where, to make it look closer to reality.
For the painting technique, I use the smudge tool alot (after putting the colors roughly onto the layer). This tool in photoshop is very versatile, the settings allow you to set up thousands of different effects
Hope this helps, have fun with trying out!