You've built your first workbench, survived your first terrifying night in the wilderness, and have probably constructed some sort of shelter against the creatures that hunt you at night. Once you've taken care of these basic necessities in "Minecraft," you'll need a furnace to smelt metals used for crafting stronger and more durable tools, weapons and armour; and to produce things like stone blocks and glass. You can even use furnaces to cook raw fish and pork, which more than doubles the healing points they provide.
Open your workbench's inventory by right-clicking it.
Move your cobblestone blocks to the workbench inventory screen so that they form a square around the edges of the workbench. It will take eight cobblestone blocks in all. A furnace will appear in the result box when the blocks are positioned correctly.
Drag the newly created furnace into your personal inventory, exit the workbench, then drop the furnace in a suitable location.
Open the furnace inventory by right-clicking it. You will see two empty boxes with the outline of flames between them.
Move fuel items to the bottom box. Coal is the best and most efficient type of fuel, but logs, planks and sticks will work if you're in a pinch. (Lava works, too, but you will have needed to have already used a lit furnace to smelt the materials needed for creating a bucket to retrieve lava in the first place).
Move the item you wish to smelt (or cook) into the top box. Cobblestone iron ore, gold ore, sand, clay and even fish and pork will work. When you have both fuel and something that can be smelted or cooked, the furnace will light automatically. It will continue working even if you walk away.
Maximise the healing points of fish and pork by cooking them in the furnace before consuming them. Cooked food should give you more than double the healing points of raw fish or pork. If you want to be more efficient, create multiple furnaces that smelt different types of block simultaneously. Furnaces can be stacked and built into walls to conserve space. Furnaces can hold a maximum of 64 blocks. If more than this amount is created during the smelting process, the furnace will stop producing more material, so empty it periodically. Remove each item of cooked fish or pork before the furnace can cook anything else.
You can pick up a furnace by hacking it with a pickaxe until it breaks, but empty the furnace first so you don't lose any items inside.