Before you start you journey toward the Blast Furnace it's worth noting that it's only available on the planets 352, 355 three, 358, 386, and 387. After you've spoken to the Giant Dwarf and entered Keldagrim the next step is to go north and make the crossing. Once you've crossed it, continue to go south until you see an Blast Furnace icon on the map. Once inside the designated building go downstairs, and you'll be able to see the furnace.
After you've done this then it's much easier to visit the area to visit in the near future. There's a way to make use of it by taking the Grand Exchange shortcut on the northwest wall, or take the minecart that runs through Ice Mountain Dwarven Mines, or utilize the minigame group finder to be able to enter the building.
It may take some time to get used to at first but playing the minigame on the Blast Furnace is simple. To utilize the furnace to operate, follow these steps. Drop your ore onto the conveyor belt. Then, run down the ramp with a bucket full of water or gloves made of ice. Then cool down your bars and deposit them.
Speed is key, so make sure to wear gear that helps reduce weight as you travel through to the Blast Furnace. If you're a player less than 60smithing the blast furnace, you'll need stamina potion, and 25 ore, while also paying your fees per 10 minutes.