Fire

A fire is simply logs lighted with the Firemaking skill. To make a fire, players need a log, which can be obtained through the Woodcutting skill, by cutting trees. Then they need a tinderbox. Tinderboxes are usually sold in general stores for 1 coin.

Lightning
To light a log, players just need to use tinderbox with a log, which can be in inventory or lying on ground. Other way is to right-click a log and select the 'light log' option.

After lighting a fire, player automatically walks one step to the west. If the way is blocked, they will instead take one step east. If both ways are blocked, player will go south. If all three other ways are blocked, player will step to north. A fire cannot be lit everywhere, such as inside buildings, or when standing on objects like flowers.

If you light one fire and have a clear path to west for more fires, you can immediately light the next fire by using a tinderbox to logs just after you have finished the lighting of first log. This way the fire will start burning before the player even attempts to light it. This is much faster way to burn logs that burning each one separately. It's called power firemaking.