Note:I recommend you have at least 200k before attempting this.
Coal Merchanting
Go to a busy local world (1850-1950 people) and yell in Falador bank, “Buying all coal 150 each” until you have 1k+ coal.
Then, go to the forums or world 1 and sell the coal for at least 180 each in Falador.
Iron Merchanting
Go to any busy local world and yell in Falador bank, “Buying all iron 75 each” until you have 2k + iron.
Then, go to the forums or world 1 and sell the iron for at least 100 each in Falador.
Iron Bar Making
Note:You need to be a member to profit from this.
Get many rings of forging and buy some iron (at no more than 100 each). Next, equip your ring of forging and smelt iron bars in Al-kharid. Note that each ring lasts for 5 runs. When all of your iron is turned into bars, go to forums or world 1 and sell the iron bars for at least 180 each.
Steel Bar Making
Buy coal at no more than 175 each and buy half the amount of iron at no more than 100 each. Next, go to Al-kharid and smelt your coal and iron into steel bars. When they are all gone, go to forums or world 1 and sell the steel bars for at least 580 each.
Super Heating
Note:You need 55+ mining and some natures for this.
Equip a fire staff and have a pickaxe and go into the mining guild and mine coal and mith. When you have 4 coal and 1 mith ore, use superheat item on it and sell the mith bars in world 1 or forums for no less than 1k each.