There are two types of heads for propane torches:
The one on top gives a 'pencil' flame: thin, focused, very good for some things but not what you want here. The bottom one gives a much wider flame: uses more fuel, but heats a wider area, and that's what you want.
With the assembly, er, assembled, it looks something like this.
Light the torch, work the head into place (in this case it's pointing a bit down and toward the back), and let things heat up a few minutes. This really ought to be in the shade, but the trees haven't leafed out yet; shade lets you see the inside better.
For instance, you can see the hot spot formed on the brick opposite the torch.
I stuck a piece of 1/4" diameter round steel in, and by working it back and forth in the hot spot, I was able to get about 2" of length to a low red heat in a minute or two. This wsa high enough to anneal high-carbon stock, but not really hot enough to forge, so I went to the next step:
If you can use an oxy-acetylene torch (this is a small one), you're in a whole new class. These use both an oxygen and acetylene bottle, and with with a rosebud tip (think of the 'wide' flame tip on the propane torch, supercharged) you could get real heat going inside the chamber*. Alas, I don't have one.
What I ended up doing was taking this small blade that's previously been forged and ground and worked it in the chamber. It's wider but thinner than the 1/4" stock, 3.5" long, and by working it back and forth in the sweet spot, I had no problem getting it hot enough to harden. This means I could have gotten the stock that it's made from (a broken epee blade) up to forging heat.
That's the basics of using one of these. Depending on the torch(es) you have, you can adjust things for what you need to work on, so don't be afraid to make the chamber narrower or more shallow to suit your needs.
*These are more expensive than a acetylene torch, and you'll have to get both the oxygen and acetylene bottles filled. They also can be used as a cutting torch and for brazing as well as a general heating tool.