up on running water through the plenum while the engine is running. I've never done it myself. If the plenum is removed: I used solvents (acetone) and long nose pliers and rags to reach into areas and scrub when I removed the upper plenum on my 94TT. Took about 3 or 4 hours to clean it up.
FYI...I was warned by another member to not mess with the plastic parts on the throttle linkage that connects both sides of the intakes. The plastic parts break easily and it is hard to find replacement parts.