You can pour it anywhere. Depending on what you need.... the can gives all the directions, but it can be poured: in the crankcase, into the intake, in the gas tank, and more.
It does work when poured into the tank with the gas. But it works much better if poured directly into the intake. If you have a a throttle body you can just pour it down the throat, if your car is MPFI you will need to find a large vacuum hose, such as the booster hose to pour it into. You actually pour it in and let it soak for a while then crank up the car. It will smoke like a freight train, but wow does it help.