This is the inside of the switch box. It's a metal box. With 3 holes. Of which you can only see one.

One hole is in the back, and the other two are in the ends. I only needed the two end ones...

Because that's apparently a common problem, they give you two plugs for unused holes, plus some mounting screws, some metal things I couldn't find a use for, and a green screw to ground the box. (more on that when we get there)

Problem solved.

Next I moved about 10 feet from the plug end of the cord and got ready to operate. (you can also see the switch plates that I bought but neglected to photograph. I got one 4 outlet one for the plug box, and a solid metal one that I drilled a hole in for the dimmer switch, because I didn't want all the empty space around the pole that you have when you use an ordinary switch cover.

I had every intention of just opening the cord, splicing in the dimmer, and trying to be all neat about it, but then I realized that the plug won't fit through that hole, so I was forced to cut it in half.

Electrical lesson #1. Look at an outlet. See the 3 holes/slots? The longer one is the return. The shorter one is the hot, and the round one below them is the ground. Look at the picture. See the 3 wires? Guess where they go. That's right, to the 3 holes. The white is the return (also called the neutral), the black is the hot (the powered line) and the green is the ground.

I ran the newly plug free wire through a clamp and into the box. I did the same with the other end on the other side.

I inserted the dimmer switch into the hot (black) line (wires). If you aren't sure what a dimmer does, it blocks some of the incoming electricity, so the load at the other end, (load: thing plugged into the circuit using the electricity...a light bulb, in this case) gets less power, and therefore is not as bright. Basically I used a wire nut (the yellow connectors) to attach one black wire of the switch to a black wire in the cord, and hooked the other one to the other half of the cord. Then I hooked the whites back together.
Now the incoming electricity from the extension cord has to go through the dimmer before it can continue down the wire, through the load and back through the return. Electricity has to travel in a loop in order to do work. It starts at the generator, comes in on the hot wire, and goes back to the generator through the return...essentially a big circle.
Finally I hooked the grounds back together, but first I had to run another wire off of the ground to attach to the ground screw in the box.
Another Electrical Lesson: Grounding is important. Essentially a ground is a wire that is ultimately connected to the ground somewhere. (Technically that's also where the white return wire is connected, but don't worry about that) The ground wire is a second return... an unpowered wire that doesn't actively take part in the circuit. It's sole purpose in life is to sit around and wait for the off chance that a powered wire comes loose and touches the side of the box. (or, more likely, if you messed up when you wired it.) Since it's metal, if you were holding it when that happened, you would be zapped. If it's grounded, however, when the wire touches the box, instead of zapping you, it'll get bypassed into the ground circuit (because it's the path of least resistance) where it'll blow the circuit breaker and keep you alive.

I should have bought the red wire...there's too much yellow.

After two tries, I made it all fit in the box.
Another electrical lesson...If you have a whole spool of wire, don't be stingy. Use pieces of decent length. It makes it so much easier to fit it all in the box if you can move it around inside. I skimped, and as a result it wouldn't fit, so I had to open it up and replace the short pieces. :-(

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