Craft a furnace:
Open the crafting table (3×3 grid)
Place 8 cobblestone in the outer squares (all slots except the center)
Place nothing in the center slot
Take the furnace from the output slot
Alternative (smelting):
Use a furnace to smelt items:
Put fuel in the bottom slot
Put smeltable item in the top slot
Wait for the output to appear on the right
Common fuel options:
Coal
Charcoal
Wood logs (some versions)
Planks (some versions)
Place the furnace:
Select the furnace in your hotbar
Use/place it on a block in the world