Runescape updates are a great option to invest. This is one of my favourite methods to start. You can expect antifire potions to RS gold increase with the QBD or new KBD drops. Therefore, you'll need to purchase a lot of them.
For player owned ports you may want to buy planks because players may want to train on construction. Bolts and arrows are possible investment options for the ranged-slayer Dungeon. These are only a few examples of how you can buy OSRS gold make use of these updates. Reading graphs comes after some time. While it may sound easy to figure out the best time to buy an item, or the best time to sell it, it's not always straightforward.