Below is a quick series of steps for preparing a simple mesh which will work well with displacement painting, particularly for relatively spherical objects such as a head.

1. Create a cube.

2. Subdivide two times. (Press C twice.)

3. Refine the subdivision. (Press Shift + C.)

4. If you desire the model to be symmetrical, Calculate Symmetry. (Press N.)

5. Press Shift + T to enter the displacement tool and open the Brush Editor.

6. You're ready to go. Switch between subdivision levels using Subdivide and Unsubdivide, and paint using the brushes available in the Brush Editor. See the help files and guides on displacement painting for more information on how to use the tools.