Tutorials/Automatic smelting

What you need (minimum):

 * 3 Chests
 * 3 Hopper
 * 1 Furnace

What you need to make the materials above (minimum):

 * 8 cobblestone
 * 15 iron ingots
 * 48 wooden planks

How to build an automatic furnace:

 * Step 1: Place down a chest on the ground. (This will be the chest in which the smelted / cooked items will be put)
 * Step 2: Place a hopper on one side of the chest (sides or top)
 * Step 3: Place the furnace on top of the hopper.
 * Step 4: Place 1 hopper on the TOP and 1 hopper on the LEFT SIDE (important) of the furnace. (The top one will insert the items into the furnace, which get smelted / cooked; The hopper on the left side inputs coal into the furnace)
 * Step 5: Place a chest on top of each hopper attached to the furnace.

Video
The only downside to using this method is that you won't get any xp.