During your visit to Saturn Valley, a Mr. Saturn told you the password to enter Belch Base.
Enter Belch Base
Now that you know the secret of getting into the Belch Base, go back through the short cave and go north to Grapefruit Falls. Step through the falling water and walk left until the belch guy asks you for the password. Then do what Mr. Saturn said: stand still and wait for three minutes. This means that after you press the button to get rid of the "say the password" dialog box, don't press any buttons until the belch guard talks to you again. He'll say "you may enter," and you'll automatically go into the base.
Go through Belch Base
This place is gross. You NEED the fly honey here, so be sure that you have it. You'll be accosted by a slimy little pile near the door who will fight you if you don't have the fly honey. So go get it if you don't have it.
Some of the rooms in here have trash cans with useful items inside. Go ahead and get those. There is also a Magic Butterfly room somewhere in here.
As you make your way in, there are places where you will be ambushed by large gangs of Foppies. Don't despair if a huge group of them attacks you at once. They're really easy to defeat. They also have a surprisingly large amount of EXP. Note that once you defeat the boss here, they will go away, so use the Foppies for some good level building before that happens.
Defeat Master Belch
After passing by the enslaved Mr. Saturns who are being forced to make more and more fly honey for Master Belch, you will soon reach the nasty pile of goo himself. He'll burp a lot while saying mean things to you. Then he'll attack. Find out how to defeat Master Belch.