Step 2: The Chamber Hemispheres
The shaping of the two halves of the chamber was, for me, the most difficult part of the entire process. It requires a lot of patience, and there is a significant learning curve. After I saw that my second one was so much better than my first one, I went back and tried to fix up the first one. It kind of worked. What you are seeking to achieve is two equally sized (especially in diameter, but also, for aesthetics, in depth) hemispheres that protrude from the FLAT metal. This is important because the two sides are soldered together later, and the smaller your gaps are the less irritating that step will be. Anyhow, on to the step by step part:
First, if necessary, cut your steel until you have two 12x12in squares (this is a little large, but personally, I needed the room to work with).
On your bag of charcoal (or sand, or dirt, or gravel, or whatever), place on metal sheet. Find (roughly) the center, and hit it with the ball end of the hammer.
Work in generally concentric circles, continuing hitting the metal until you have an indention that is slightly smaller than how large you want your chamber to be. IMPORTANT: your chamber can only be as large as the largest distance your vise can open.
Open your vise to the diameter that you want your chamber to be, place the indention within the vise, and begin to hit it with the hammer again. Work in one area, slowly rotating the piece, so that you end up with a perfect circle.
Repeat for the second hemisphere
Optional (but HIGHLY recommended): at this point, trim the excess metal from your hemispheres. I used old, super heavy duty aviation snips, and left about 3/4 of an inch of metal. However, I did this AFTER soldering the tubes into it, which made it more difficult than it needed to be. Another note about aviation snips: they exert torque when they cut, which, in mine, led to warping of the metal. I had to go back to the vise and hammer it flat again.