If you have a decent double action air brush, try free handing it. Here is what seems to work best for me:
- Use highly thinned paint
- Get in really close with the airbrush
- Keep the airbrush moving at all times, even when paint isn't actually coming out
- While you're moving it around gently and quickly pull the trigger back ever so slightly, then rock it forward again as a bit of paint hits the surface of the kit. This will give you light, gentle and random mottle patterns.
The key is thinned paint and gentle rocking of the trigger. If you pull back too far, you're going to get a blotch. If you don't keep the airbrush moving, all your mottles will have the same, circular appearance and you run the risk of paint building up in one spot way too quickly, thus creating runs and other issues.
Experiment with that techique on some scrap or even using a blank sheet of paper until you feel comfortable and in control of the technique. Experiment with the thinning ratios as well to help maximize the softness and subtleties of the mottling you're trying to replicate.
I hope that helps.