This technique requires basic knowledge of using Photoshop.

Web resolution screentones available here:
http://www.amethyst-angel.com/screentones/

Higher resolution screentones (300 dpi) available here:
http://www.amethyst-angel.com/screentones_hires/

For other additional tutorials, feel free to visit: http://www.drunkduck.com/Comic_Tutorials/