Um, save the electrical load on the alternator, and store the Hydrogen generator on the shelf?
(Conservation of energy, and the efficiency of converting mechanical energy to electrical in the alternator alone, loosing proposition.. )
As to how to install one, I've never seen one. I don't want to be too much of a jerk about it, but the 'kits' that are out there seem to be based on some seriously faulty logic and theories.
If you do go ahead and install it, please give us some good comparisons of mileage and emissions, before and after. (averaged over time, and the like?)
Best of luck!
(scam explanations:
here and
here)