1) Seafoam in the fuel tank is just a fuel cleaner.

2) Seafoam in the oil, via the valvecover oil cap, will help clean the bottom part of the engine BUT you have to change the oil shortly thereafter, probably within the first 20 miles or so, so it's generally only done just before an oil change. You do not want to run the engine for any long period of time while the engine oil is mixed with Seafoam.

3) To clean the upper part of the valve train, use a funnel to put Seafoam into the vaccum line from the engine, the big tube that powers the brake master cylinder. You will need to rev the engine to keep it from stalling as the Seafoam is fed to the injectors, throwing the mixture off.