Disconnect the battery by turning the positive terminal bolt counterclockwise, then taking the bolt from the battery. Store the open terminal away from the battery or metal.
Measure and run the hoses from the HHO generator to the fuel intake of the vehicle. This is usually done by cutting a small hole into the intake hose of the car and pushing the output hose of the generator into it at an angle. Some older, carburetor-equipped models can have the HHO go directly into the service vacuum port, if applicable. Locations on fuel-injected models are as varied as the experimenters, but a third possible location is a splice into the vacuum lines that lead to the fuel intake manifold. The HHO gas can be introduced to the injectors in this manner, and the lines create enough vacuum to draw the gas into the injection system. Use hose clamps to secure any loose ends.
Wire the generator to the vehicle by running the negative terminal post to a good, solid body ground. These terminals are usually long bolts topped by wing nuts, but the generator designs vary. The positive terminal wire will need an in-line fuse to protect the car from overload, and can be run to an ignition-on source of power. The generator can also be run to a simple switch installed on the dash. Common places to add this lead are the fuse panel, the ignition switch wire or an accessory port.
Wire the EFIE control box to the car's sensor system by cutting and stripping the wires, then twisting the copper ends together to loop the signal back through the box. In cars with computer systems (post-1980s), the MAF sensor and O2 sensor must be wired into a control box, called an EFIE box. This box is usually supplied with HHO kits, and is required to "trick" the computer into not registering the extra oxygen produced by the generator. This can void some cars' warranties, so check on this before installation. Wiring this box is simple, and only requires that the O2 and MAF sensor wires be spliced into the wires coming from the box. Instructions on wiring color codes will also be provided by the manufacturer of the HHO kit. Apply a liberal amount of electrical tape to the splices.
Reconnect the battery by turning the positive terminal bolt in a clockwise direction.