How do I drain the fuel tank?
Obviously, working with gasoline can be lethal. Plan your work area accordingly; this isn't something you want to do in the living room. Plan on spilling at least some gas.
If you're careful, you can skip the whole vacuum pump process (see below) and go straight to pulling the petcock.
This is about a 20 minute or so process, unless you don't follow the "use someone's car to go to the gas station" bit, in which case it could take longer, depending on how fast you can walk.
Vacuum pump way
When fuel stops, disconnect the hand pump and the fuel line, seal up your gas can (after adding an appropriate amount of Stabil, assuming you're going to store the gas), and hook up the fuel and vacuum lines to the petcock.
Here's a photo of the Jim Race not-club-sanctioned way to do it. We suggest you omit the shopping cart, and the basketball. It does, however, give you a good idea of the relative locations of the vacuum port and (larger) gas line drain.
Sealing tank openings for cleaning
The tank shown below had a bit o'rust in it. Here is how our man sealed it up for derusting so the chemicals didn't affect the lock, leak out the keyhole, or hurt the petcock.
Inlet (gas cap) plug
Use a 1 13/16" x 1 1/2" x 1" rubber stopper, in this case the largest one Lowes had in their hardware section. Fits ok if you press by hand - fits tighter after a whack with a rubber mallet.
There should be other sizes that will work, as will some bathtub stoppers.
Outlet (petcock) plug
Make a plate to fit in place of the petcock out of scrap aluminum or wood. This one is about 1 1/4" x 1/4". Space the centers of the holes at 1 3/8" using a 1/4" drill bit. Smooth off the burs and rough edges with an electric wire brush. Cut a matching gasket from cork-rubber gasket material or bicycle innertube and bolt the plug into place on the tank.