Generally, the "pyro" tribe is the better place for these questions, but here ya go:

The very basic torch, like a 'weed burner' involves [in order]
1) a source (portable bottle, 5 gal bottle, big ole tanke, etc)
2) a valve (to keep the gas out of the system when not in use)
3) [optional] delivery - flexible or rigid tubing to get the compressed gas to the device
4) [optional] backup valve - this is usually the faster, trickier, or more suspect valve. If it freezes open, you still have the main.
5) expansion area - cold resistant area that allows the compressed gas to expand into atmospheric pressure.
6) fuel/air mixing point - sometimes hidden (like in a propane torch) but is usually the point of ignition in a basic device. Typically, this is a hole drilled into the expansion area that becomes the source of the flame. Naturally, needs to be heat resistant and cold resistant.

The trickier you get with this model, or the larger your propane source, the more you'll want to consider putting in flashback suppressors, fully rated hosing, multiple fail safes, etc. Even the basic weed burner can be a hazardous tool. Also, don't skimp on any of the parts. Go to a welding supply house to get the flexible hosing, and get it fully rated for propane.

Personally, I tend to use a lot of copper plumbing tubing in my portable designs, and the heavy steel stock for installations. I only use flexible hosing when necessary (like the swords), and then only in the part where needed, and it was fully rated for propane and heavy flex-use. Everything else got full brazing and multiple valves.

My simplest device is a staff with a portable bottle on the bottom, ball valve (high pressure plumbing variety), attached to the bottle, a 4' tube (expansion) of copper, another valve (big handle for faster action, but same plumbing valve), and a capped copper tube on top with a few holes and slits cut in. Runs about 4 hours on a bottle and allows mini cannon blasts when I throw the big valve.