How do you use your guilded altar on runescape?

The spelling is "gilded altar". You take bones, or certain special ashes (popular choices are dragon bones and infernal ashes), then right-click on a bone/ash and USE it on the altar. This alone will increase the prayer experience per bone (or ash) by a factor 2.5. However, if you light both burners first (requires 2 marrentil and a tinderbox), you will get a factor 3.5 out of the same bones or ashes.