Firing up the Therm-x is easy as pie. Here's how it works on mine (which may be slightly different from yours):
Step 1: turn on the gas by opening up any shut-off valve in the line leading to the heater, and also by turning the heater control knob to any "ON" position (on mine, either LOW, MED, or HIGH).
Step 2: Press the lever all the way to the left, and simultaneously press the spark button (like on a gas grill) a few times until you see a little starter flame at the bottom. If you don't see a spark when you press the button you might have a bad piezo starter.
Step 3: Hold the lever to the left for a few seconds. This feeds gas to the starter flame and allows it to warm up the catalyst. Then release the lever, and the little starter flame will go out. The catalyst will continue to be fed propane and generate heat until you turn the control knob to "OFF".
You can actually see the catalyst glowing in the dark, or just feel the radiant heat to know it's on. Mine puts out a LOT of heat, even on "LOW".
To be safe: Keep flammables away, like bed sheets and blankets.
Don't run it while sleeping. The catalytic heater consumes oxygen and the older ones may not have an automatic safety shut-off. (Mine does not.) Always crack open a window in accordance with the safety instructions printed on the heater or in the heater documentation. And turn off the gas at the gas line when you are towing.
Oh yeah, remember to gloat when you are boondocking -- your Therm-x doesn't require any electricity!
Best of luck