Here's my gas can taken off. Visit here (link to come) to see how I did that. I drained all the gas as well. Twist the petcock bowl to remove it.
Here's what the inside looks like. It is comprised of a rubber gasket and a screen for fuel filtering
Take those two items out and this is what you're left with. Unscrew the 3 screws and make sure you hang on to the washers. I put all loose parts in a bag so they are contained.
Here's what you just unscrewed
To further disassemble, use a screw driver to gently lift this tab. The metal pinwheel-lookin' piece contacts the surface of the gas tank and puts pressure on the assembly you're about to take out. It makes sure there is no leakage from these parts in the petcock
Now you can take it all apart.
You re-assemble the petcock by starting with 0, adding 1, and continuing to 4. 3 only goes in 1 way. Remember which way you took 2 out (you could figure it out though if you forgot) 4 is the o-ring that seals the petcock body to the tank. 3 comes apart further if you want to replaces the rubber parts and the tubing.
Goes on the same way you took it off. The petcock screws into the tank in 1 orientation. All other ways make the petcock switch inaccessible.
OK I think that's it!