Good enough for an afternoon of fun or as the starting point for a bigger project, this is a super simple helmet you can make in around 15 minutes from a single cereal box!

While this design is based off the historical Viking Gjermundbu Helmet, you could easily adapt this project to make yourself any number of historical, fantasy, or science fiction helmets.

Supplies

Cereal Box
Scissors
Pen or Pencil
Stapler
Tape (optional)

Step 1: Prepare the Cereal Box

Cut off the top and bottom flaps of the cereal box.
Note: Don't throw the top and bottom flaps away just yet in case you need them in Step 2.
Draw the helmet design on the front panel, placing the location of your eyebrows approximately halfway down the box.
Draw two additional vertical lines starting approximately an inch above the eyebrows. This will become the center ridge of the helmet.
Mark the corresponding center ridge lines on the back panel.

Step 2: Initial Cuts and Shaping

Cutting

Start cutting! Cut out the eye-holes and other details on the front panel.
Cut along the center ridge lines.
Cut the corner edges of the cereal box down to where the center ridge lines stop.

Shaping

After making these cuts, squash the box to make it rounder and more head-shaped.
Slightly crease the edges to help hold the shape.
Put the helmet on your head, positioning it so that you can comfortably see out the eye holes.
Once you have the right placement, fold down the side panels so that they are snug against the top of your head.
The side panels should slightly overlap. Stapling the side panels where they overlap together will lock in the width and height of the helmet. Reinforce the staples with some tape if you don't think the staples you have are strong enough on their own.
Note: If the side panels don't overlap, this is where the top and bottom flaps you saved might come in handy: you can use them to help bridge the gap.

Step 3: Preparing the "Bowl"

The left and right thirds of the front (and back) panel will form the "bowl" or "skull" of the helmet.
Fold each bowl flap towards the center of the helmet and mark where the panels intersect. (See pictures.)
From that point, draw one line from that point down the center line of the helmet, and another from that point to where the edge cut ends.
Repeat these steps for all four bowl flaps.

Step 4: Forming the "Bowl"

Cut along the lines you marked in the previous step.
After the bowl flaps have been cut, fold them underneath the side panel bridge.

Step 5: Finishing Your Cereal Box Helmet!

Fold over the center ridge pieces and staple them into place.

That's it!
Decorate it with some details or "battle damage", use this as a base for a papier-mâché helmet, or pair it with your favorite costume beard left over from last Halloween and have fun!