I started with this site to get information. Water Car Run on WATER+Fuel. Hydrogen Save Gas=FREE ENERGY! but I ended up buying a kit from Tim Demers at Home Then I started making my own and modifying the design based on Google searches and YouTube videos I saw of different peoples designs they were trying.
The truck you saw running solely on HHO was something called a Joe's Cell. If you are interested in reading the theory behind this design look here http://www.thejoecell.com/ I don't have the skills to build one of these myself but I will say the general design they use + N - N + in the plates does in fact work very well. By far the most efficient design I tried (I have no idea why though). It pulls about 12amps per generator but produces significantly more HHO. Basically I was able to replace six generators of the old design with two of these and got slightly better mpg. There isn't a document available on making them so I just followed one of the YouTube videos of a guy describing how he made the plate design. It turned out to be very simple.