daniel,
you kinda picked a difficult one as a first go around.
For that first piece, you are making a post to contact the positive side of the battery. It has to fulfill a few requirements.
1) It has to conduct electricity
2) It has to make good contact with the positive post of the battery
3) It has to be the correct distance from *wherever* that the battery makes good contact with it and the negative contact
4) It has to insulate the positive from the negative
5) It has to be easily solderable
I don't know anything about the spacing that you need to fulfill 2 and 3. You should be able to figure that out by putting your battery in the tube and seeing how far it sticks into the upper piece.
He cut a round piece out of stiff plastic and fit in the upper piece (4). He put the spare 510 positive post in that so that it would make contact with the positive tit on the battery(1,5). I won't bother telling you where to find a thick plastic top from a wide mouth bottle. For the spare 510 post, you need to find something that can be easily soldered and provide that center contact piece. You could use a brass screw, a brass or copper rivet, a metal thumb tack, a coin, roofing nail, etc. Copper and brass will be easiest to solder. Steel should be doable with what you have on hand. You could even not use solder. A short round head screw, a nut and a spade connector would get you connected, too.
So, you make your plastic disc that fits tightly into the top piece. Then you drill a hole in the center of it that is as large as the post on whatever metal piece you decided to use or big enough to pass a wire through if you decided to use something flat like a coin. Then you solder your wire to your metal piece and glue your metal piece in the center of your disc.
When done, you will have a flat plastic disc. On one side, it will have the metal tit you glued to it. On the other side, it will have the wire coming out that you soldered to your tit (ouch!).
BTW, Gorilla glue is good for some specific application. I just haven't figured out what that application is, unless it is semi permanent stains on skin.
As far as wire, make sure you get STRANDED wire. Available at radio shack
I hope that sheds a little light. If not, ask away.