39sec after effects video tutorial. Enjoy this =)
For complete this stuff, first, you need clouds Photoshop brushes, then you must create several layers in Photoshop and paint different clouds on each. Then import PSD file as composition in AE and you can see all of your cloud layers. Then, convert layers into 3d layers and distribute it belong Z-axis. Thats it. Then, you can create camera and animate it's flying through clouds layers.
By the way, for more realistic clouds, you can use some kind of particles software like Particular or similar.
music — Air