I'm familiar with what a sling is. you can purchase it from cows too. Second cow kill, I received an sling. The first kill was an air staff. Questing is another issue, I am a snob at questing. I have no idea which runescape gold quests to take on and if they're long, I get bored easily. Therefore, questing for full rune armor and melee training are the only issues I have.
You can't just transform money into more. Just run herby or train for profitable skills such as hunter longs, fletch yew, anything similar. Someone suggested 99 fishing, this is probably the way to go but make sure to fish rocktail 90-99. Sure, you can, but it is slow and long dependent on the point at which you begin and how dedicated you are it is possible to turn 1gp into 1B within a short period of time.
The way I would do it even though you've 15 million is to invest in something that fluctuates in price , but is held at the bottom by its alch value; at this point , you do not want to be taking big risks when you invest your money however, building it slowly. 15 million which is a decent amount will gain you about 400k in profit per cycle of green chaps, for instance.
Since they aren't able to drop to below 2200 in a very long time since they have an average alch of 2380,, you could buy them for 2200-2250, and sell them for 2300-2350. It's not necessarily the most effective method out there but it's consistent. if you are a member you are able to purchase higher priced items, like black chaps, buying at 5400 and selling at 6000+ , but if don't plan it correctly, it could mean you hold stocks for quite a while.
Avoid red dragonhidesince it's not very popular and also avoid seeds like they're a plague. I've lost a fortune in them. The way to do this is to access the graph over 180 days and examine the bounces constantly between two points and purchase and sell before they meet, it requires some time and practice, but once you get it right, you can make enormous profits of 1-2 million everyday...do some slayer or woodcutting simultaneously so you're not wasting time.
Herb running (in definition) is the running that runs between all of the herb patches , usually in the most time efficent method. It takes time to perfect an effective route that runs among the ones in the osrs gold site undulating water (forget its name) that is near draynor manor, the one north of ardyand the one in Catherby, and (if the quest is done) the patch up in Trollheim.