basically you take a jar(or multiple jars for better results) put an anode and a cathode in(stainless steel wire works best) and put electricty to it, it splits the water into H and O and pipe it into your intake(if you have any kind of forced induction put it before that to it still pulls a vacuum) for a diesel you're done, for a gas engine you need to trick your sensors to use the hydrogen more effeciently.
you can use propane in gas engines as a fuel by itself LPG (liquid petroleum gas) is what its called CNG is also used its a little different but close some new hybrids are CNG and honda is coming out with a CNG car its really neat stuff i would do a propane kit in my car if it wasnt for my stereo, but you dont have to change to much just add stuff and you can switch between gas and propane while you are driving here is a website that does the LPG and CNG kits http://www.lpg-kits.com/ most of the companies doing this to gas cars are oversees and it takes a week or 2 to get a reply to your emails, ive only heard of a few people doing it to their cars here i think it is definitely worth if for environmental reasons im not so sure for fuel economy