Source: Wikia
Farming Big Bones and Brass key to earn Big RS gold
Bones are usually use with Prayer skill. Big bones currently give the highest XP of any bone excluding the quest-reward only Ancient bones which you can Sell RS gold. When big bones used in Player-owned house on the Gilded Altar, big bones produces 37.5 XP and it can be increased when worshipped at the Ectofuntus producing 60 XP each big bones.
First it is required for you to get one Brass Key from the Grand Exchange which will require you to use RS gold in order to enter the place you are going. Don’t worry because your RS gold will be doubled once we finish looting big bones and Brass key.
Go to the west of the Cook’s Guild on the way to the little house, slightly to the south-west of Grand Exchange. Under the place, there are two way to make good RS gold.
The first one is to kill all the Giants and loot all the big bones which we will be using to make RS gold. The other one is to go straight from where you entered and to the right on the first room. Ignore all the zombies and just loot all the Brass keys on the ground. The Brass keys will respawn quickly so you can fill up the inventory space.
By following this guide, you can make a decent amount of RS gold. You may repeat the process until you are satisfied.