There are a total of 10 fires to light - 4 of them are for the progression of the main story (located in the throne room) and 6 are in the main starting area of the Fire Kingdom. Pick up one of the cauldrons and travel to the starting area of the kingdom (where Cinnabon is located) They are not hard to find and they are are all located only in the start area of the kingdom. Walk next to the unlit cauldron to start the fire. Once the last of them are lit, the achievement pops.
NOTE: If the achievement does not pop, it may be due to the last BLUE flame cauldron to light - I did not include that one in my above count. I had went and did the 4 in the throne room AND then the blue flame before heading out to the starting area to light the other 6.