Brent Williams of SpeakWisdom (we met him earlier on the blog) has started a YouTube tDCS series. This is his second in the series. This is an excellent overview of the basic components of tDCS, however, Brent does not recommend you actually build
and use a resistor-based device. Brent mentions that upcoming videos will demonstrate how to build a current-regulated device.

http://www.youtube.com/watch?v=d9iId0Aen2g