Most people who paint in watercolor must surrender themselves to the medium. Watercolor is difficult to control, and the results are typically unpredictable. This is why watercolor is often used to paint landscapes, flowers and other organic objects that do not need to be depicted precisely in order to look right. On the other hand, human subjects can require precise handling in order to look as they should. In this case, watercolor must be used carefully in order to achieve optimal results.
Preparation
Draw the subject on the paper only very faintly. Watercolor is applied to the paper in transparent layers that build on themselves. If you must draw the subject before painting, then make the pencil marks very lightly so they will not show up beneath the paint.
If you are working from a photograph or a live model, you may wish to make your drawings and studies of the subject before beginning the painting. This will make your final product more precise.
Application
Apply the watercolor to the paper in very thin layers. Watercolor is difficult to control, but humans are generally very precise subjects to paint. Especially if your subject is featured close up, as in the case of a portrait, applying the paint to the paper in thin layers and building color very slowly will allow you to exercise control over the medium.
Process
Keep in mind that there isn't a good way to correct a mistake made in watercolor, especially once the paint has dried on the paper. Make your choices deliberately. Work slowly, patiently and with care.
In fact, watercolor is such an odd, counter-intuitive medium, you may not wish to begin with human subjects if this is your first attempt at using watercolor. The best introduction to watercolors is landscapes or simple paintings.
