Some people use a full can, others 1/2 a can..
you use a vacuum line to "suck up" the seafoam into the engine.. You have a bunch of vacuum lines on the drivers side of the upper intake (plenum).. connected to an octopus looking things.
Some like to use the Brake booster or cruise control hose but those are large and you can suck in so much to stall the vehicle (could possible do damage if sucked in way too much).. Using the smaller hoses (or just add a small hose to the ocotpus when your doing the seafoam) will give you less of a chance of putting too much in...
Search around here, there is other tips in the many seafoam threads from people using it.
~Mark