Fuel venting FAQ page is up.
It lists vented fuel tank capacities, how to vent on the mk3 and mk4. Note that mk5 is not included because they don't use this vent removal. I also mirrored some other pics to save them.
Venting lets you add a few gallons more of fuel into the tank, extending the range per tank. I try to avoid rating articles as suggested difficulty: 1/5 because there's always some way to screw it up, but venting is fast and easy. Just make sure to clean the area around the vent and keep dirt out of the fuel filler neck.
Article can be seen here: