I usually start with eye candy, then mask the area I want the smoke to be when it is done. I use the smear brush lightly, and a lot of airbrushing to tweak it till it is exactly like i want. I start painting by drawing squiggly little lines flowing in the direction I want the smoke to waft about, then build on that by airbrushing smoke colors, then smear it up a bit. All the smoke in the above pic is done that way.