See: Idle Air Control Valve Cleaning... Problem.. - Ultimate Subaru Message Board
for cleaning.
Get a new gasket before you take it apart and the old one is unreuseable. The screws are in tight. PB them first. I just use an impact sxrewdriver. You'll have to remove a few hoses to get to it. Make sure the large hose is reattached to the snorkle, or it'll die on you while trying to start it.