I recently learned this from Fred Hind's ( we did adjustments at Mystic Springs for the vrey reason you mention).
Light the heater, you know how to do that now.
Then there is a slotted tube on the main gas inlet. There should be a screw, loosen it. Then slide the slotted tube (shutter) left or right to adjust the air/propane mix. The tube I am describing runs from the thermostat into the hole in the heater.
It should be mostly blue with a little yellow on the tips and no roar.