Hello: Flue cleaning is not hard but it is dirty. You’ll need a shop vac. Turn gas to pilot. Remove draft hood and vent pipe. Hose out the vent pipe (outside!) Pull baffel up and out, but in doing so, rotate it in the flue, bang it around a little. This will help clean off the soot which will fall down onto the burner. Take baffel out and hose it off. Water does a nice job of getting rid of soot. Vacuum out the combustion chamber. If you want to get technical, remove the burner assembly and carefully hose out the burner assembly ABOVE the main orifice. The point being to clean any soot out of the steel/cast iron burner, while keeping the rest dry. Otherwise, just vacuum it in place.
If things go a certain way, you’ll look like a coal miner when you’re done, but the heater will burn with a nice blue flame 😎 If it does not, gas pressure is the next thing to check.