Do you not want to constantly have to keep checking on your furnace to see if you can put your next item in? Are you too lazy to manually put coal or food in the furnace? Well this is for you!
This requires 1 furnace, 3 chests, 5 hoppers, 3 ladders, 1 comparator, 4 signs (optional), glass pane(optional), and select block of choice (multiple of them)
Step 1: Chest, Hopper, Furnace
Place a chest, then put a hopper on the back of it. On top of the hopper, place a furnace.
Step 2: Hoppers, Hoppers, Hoppers!
Place 2 hoppers on the side of the furnace one leading to another. On top of the furnace, put a hopper facing down. On top of the hopper leading into the hopper that is leading into the furnace, place a hopper facing down
Step 3: Chest, Comparator, Block, Light
Place the last two chests on the hoppers facing down. On the one leading directly into the furnace, put a sign on it that says "Raw Items", The other chest says "Fuel". On the bottom chest, place a sign on it that says "Cooked Items", and have a comparator leading out of it into a block. Place a Redstone Lamp on top of the block.
Step 4: Front Cover
Add a cover to the front, exposing the redstone lamp, furnace, and chests.
Step 5: Roof, Back Sides
Cover the rest with the block you chose, exposing nothing but the chests.
Step 6: Ladders, Details, Done.
Add a ladder to the top of the Auto-Furnace. Put a glass pane in front of the lamp, and place a sign that says "Auto-Furnace" and you are done. Have fun being lazy/saving time!