The method is described in full in the 'Brush Painting Guide' I posted some time ago. The trick is to use the cotton bud variety, rather than the foam sponge type, and dab it on to a piece of paper before use, in order to remove some of the paint.
Read the guide and it'll show you how to go about it, including a final diffusing process.
Here's a pic of a 1/48th scale '109 I did this way about 20 years ago. However, if you have an air brush, you'll get a much better result - use a low pressure, get in close, and build up light mottles.