Decarboning an engine can be accomplished many ways.
Marvel Mystery Oil is an old standby, Chevron Techron fuel additive is the latest and greatest. Shops often use a quick clean decarboning fluid. Car manufacturers each have their own, available at a dealership.
Do a Google search for abundant reading material.
Water injection has been around forever. It is primarily used on high compression, turbocharged, and supercharged engines to eliminate detonation and increase fuel charge density. As a byproduct, the steam tends to "steam clean" the inside of the engine. The common backyard mechanic method of implimentation involves putting a clean water/alcohol mixture in the windshield washer tank and feeding it into the the intake through a spray nozzle while running the engine at 1500-2000 RPM. This was easy on carburated engines but is trickier to accomplish on fuel injected ones. Many companies sell water injection kits. A little searching and reading would give you ideas on how to do this yourself. The water may be simply sucked into the vacuum system, a method not appropriate for driving around, but you still must limit the ammount of water with a spray nozzle or oriface.
