There are many methods out there for creating digital smoke. I stumbled upon this particualr method while trying to get a soft water effect. I adjusted it a bit and the result is this technique for creating smoke.
The brush panel settings used to paint the initial white lines are as follows: I start off with a soft edge round brush (hardness set to 0%). Airbrush and Smoothing are checked. Other Dynamics is checked with the Opacity Jitter at 0%, Control is off, and Flow Jitter at 0% with Control set to p