Creating clouds? There are a few ways to approach them. You might immediately think that billboards might be a great and fast way to go. If you need some more volume to them, then perhaps a smoke or fluid simulation. What about volumetrics though? You can probably get the cloud look that you need out of using volumetrics in Blender.
Key to the effect is being able to define gradients on the volumetric effect. Watch Blender Add-on developer, shams kitz walk through putting vertical gradients on volumes. There really isn’t a lot to it. Basically it’s a simple gradient blend and a noise texture. It’s just not applied in the normal manner.