A while back I made a cardboard Captain America shield, and it's really awesome. So I thought Thor should join the party, since I'm not quite ready to make an Iron Man suit out of cardboard...
This is my first ever Instructable, by the way.

Step 1: Materials

1. Cardboard tube for handle (could be a toilet paper or paper towel roll for lower quality, or a cut to correct length poster shipment tube for a sturdier handle. Whatever tube you use should be cut to about a hand tall, though this can be altered to your liking.
2. Cardboard for head of hammer ( a box would be better, but I couldn't find any of the right size. The head should be about one handle across and 1/2 handle top to bottom.)
3. Box cutter or X-acto knife for cutting cardboard to size.
4. Duct tape, spray paint, Sharpies, or whatever you want to use to decorate your hammer. Or nothing at all.
5. Computer or phone for viewing Instructable on (that should be a given).
6. Optional: Music. I don't care what kind, I don't judge.
7. Optional: Tic Tacs or other candy. In case you get hungry.

Step 2: Make the Head of the Hammer Part 1

If you don't already have a box to use as the head, you gotta make a head now.
1. Figure out how tall your handle is (using your 3D Avengers ruler, of course).
Mine was 7 inches.
2. Make 4 rectangles on the cardboard that are 1 handle length by 1/2 handle length (in my case, 7 inches by 3.5 inches).
3. Cut out those 4 rectangles (this time with your metal yard stick).

Step 3: Make the Head of the Hammer Part 2

Now, you need to make 2 cardboard squares that are 1/2 handle length by 1/2 handle length (in my case, 3.5 in x 3.5 in).
1. Mark the squares
2. Cut the squares

Step 4: Put It All Together Part 1

Now, you need to combine the 6 cardboard quadrilaterals into one rectangular prism. It doesn't matter how you do it, as long as it'll stay. You can do duct tape, neatly or messily, since you're going to cover it up with more tape later. You can use Elmer's glue or Super glue or paste or hot glue. You can use magical powers to put them together. You can use your grandmas secret recipe for glue. You can use peanut butter. Just get them to stick together.

Step 5: Put It All Together Part 2

Attach the head of the hammer to the handle with your choice adhesive.

Step 6: Decorate!

This step is totally up to you. You can make your Mjølner realistic, or neon, or sparkly, or penguin-y, or just plain Joe brown. It's up to you.

Step 7: Attach Strap

To get the right look of Thor's hammer, you need a wrist strap. I made mine out of duct tape following these steps:
1. Cut a piece of duct tape 27 cm long. (It looked like the right size.)
2. Fold in half "hotdog style".
3. Fold in half "hamburger style" to form a loop. Connect the two ends with tape.
4. Ensure the loop fits on your wrist.
5. Tape the taped side of the loop to the inside of the handle.

Step 8: Enjoy!

I'm pretty satisfied with how the hammer turned out, and hope you are, too. If you have any questions or suggestions just leave a comment, I'll do my best to answer.