Before you begin this technique, make certain that you equip every slice of weight reduction gear that you have (like the Graceful Outfit) - you'll need to be as light as possible (less than 10kg on your own equipment should be enough). Now that you're properly geared, go over to Pollnivneach by taking a Magic Carpet at Shantay Pass. Following your arrival in the village, then start running south to the Agility Pyramid. There you may find NPC Simon Templeton.
Talk to him regarding the Pyramid Top and after the dialogue go to the Pyramid Program. In case your Agility is over 51 you shouldn't have a lot of problems with obstacles. In case you have managed to reach the surface of the Pyramid take the item lying there and head back to the NPC that you have previously talked to. Now he will pay you 10k gold for every Pyramid Top that you bring him.
To start off you want to get in the Great Kourend which could be retrieved via boat in Port Sarim. When you reach the interface, talk to Veos who is standing south of the inn. From there you should travel west until you find Kourend Castle. In front of the castle gates there's a significant obelisk. Inspect it to get in the cave. You then would like to run south till you see a crack in the wall - squeeze through it. Now go west and you are able to battle Twisted Banshees there. There is a safe spot from where you can shoot Banshees which is situated in the south of this area.
Even though you won't make as much money here as with other methods, it lets you also train while making some extra gold. In addition to that, be sure that you bring some Top Alchemy runes in the event that you're able to cast that spell since they fall items such as Battlestaves and Adamantite Shields.