Getting the most from the Blast Furnace is vital for your armaments but it does need a few steps to be done prior to. Be sure to stock up with GP because the next tasks won't be cheap: Start The Giant Dwarf's quest to unlock Keldagrim RuneScape Gold. You'll need at minimum 60 smithing (with none of the boosts) or, alternatively, pay $2500 towards an Blast Furnace foreman to smelt bars for 10 minutes. Every hour at the Blast Furnace will use up to 80,000 GP therefore, be prepared.
For a lower cost to enter I would suggest acquiring an entry fee reduction by acquiring the Ring of Charos (a). However, avoid using the option to pay with the Blast Furnace foreman as it will negate the value of the ring. It is possible to acquire this ring through The Creature of Fenkenstrain questline.
The addition of Ice Gloves in the Blast Furnace will be beneficial when making use of the bar dispenser since it cools the bars in a short time for you to grab. This will save time, instead of having to collect buckets of water in order to cool your bars (and it'll free up space in the inventory too!).
Before you start getting to the Blast Furnace itself it's important to know that it's only accessible within the worlds 352, 355 3, 358, 386 and 387. Once you've spoken the Giant Dwarf before advancing into Keldagrim it is necessary to take a northerly direction and then make your way across the bridge. After crossing it, continue heading south until see your Blast Furnace icon on the map. Enter the building marked runescape 07 gold, walk downstairs and you'll find the furnace itself.