Roman candle. No seriously, best answer is go to the Atwood website and see if you can download the instructions. Short answer is:
Turn gas on at the bottle.
Turn water heater knob to "on" wait a couple of minutes for air to leave lines and for gas to escape that may have built up in the burner tube.
Turn knob to "Ignite" and put one of those barbecue lighters in the tube, you'll see where the gas is supposed to come out. this should be right next to a little probe looking thingy - this is the thermocouple. You must heat the thermo to get the gas flowing to the pilot light. You have to hold the knob in "ignite" mode, because it is spring loaded. Hold this there and the flame at the pilot for about a minute. Pull out the lighter and the flame should be going from the pilot. Let go of the know and if it remains lit and then the heater burner ignites (stand back, for god sakes, man!) then you're good, otherwise, repeat until it works or you give up and the water heater gets repaired.
Originally Posted by mello mike
I have no documentation with this thing. How do you start it? There's no switch inside so it appears to be a manual start. Do you light it with a match or sparker?