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Depends what you are trying to clean. You can put it in your gas tank and run it through your fuel system, or you can also put some in your oil. Just follow the instructions on the can and be sure to put the proper amount in (obviously if you put in you're oil, don't put a lot).
Don't put it in your brake booster hose. He's just screwing with you cuz you asked a question that can be answered by looking at the seafoam can. Dump it in your gas tank.
quicklook isn't pulling anyone's leg. I pour seafoam in the brake booster hose all the time. That's the best way to clean carbon build up from the intake manifold and combustion chambers. If you look on the bottle, it says pour into the throttle body. On our trucks with the horizontal throttle inlet, it's easiest to let the seafoam get sucked through the brake booster vacuum hose.
When you pour into into the brake booster hose do it slowly. Too much at a time is not good. But, if your stuff is dirty you will definitely know when it starts to run through and you throttle up.