How do you light a oil lantern in runescape?

You use a tinderbox with it

You take some Swamp Tar to the Chemist in Rimmington. Click and use the Swamp Tar on the lamp oil still (the small one, not the big one outside his house) and dip the lantern in to collect the lamp oil you had just made. Then finish the job by lighting the oil with a tinderbox.