This is a simple and easiest to make more RS gold which it involves collecting ashes from a burnt out logs or from the Fire Spirits
Source: memegenerator
Cooking the logs and sell the ashes for RS gold
Well not really cooking but were burning the logs to get the ashes and sell it for RS gold. Ashes appear when a fire burns out from items such as wooded logs. You can also get the ashes from Fire Spirits. Fire spirits sometimes appears while adding logs to a bonfire. When the spirits appear, you can click them to gain rewards which are based on what type of logs used. Below are the other obtainable rewards from Fire Spirits (there are still other rewards you can get from fire spirits):
Source: wikia
To start making fire, head to the Grand Exchange and buy a lot of cheap logs (like willow logs) or you could just start cutting down a trees. Fire the entire logs and wait for the fire burns out. Pick up all the ashes as many as you can. Sell them at the Grand Exchange for Rs gold. Rinse and repeat until you’re satisfied.