Cardboard cube that fits on your head
2nd cube that just fits around the first
X-acto knife (or other type of knife)
The knowledge of what a face looks like
4 (or so) sheets of black mesh/see-through cloth/etc.
Tape (or whatever) for the mesh
Paint (if you want)
Step 1: Remove the Flaps
Step 2: The Cover
Secondly, you have to remove a face of the cube. It can be any one, just as long as it's not the one opposite where the flaps just were. When you cut it out, be sure to leave a small border to hold the first block in place. Save the face for use in the next step.
Finally slip the cover over the original box. If the cover extends lower than the box, cut the bottom of the cover to match up the ends.
Step 3: Keeping the Thing Still
Place the sheet about 3/4 of the way in or just so the box stays a healthy level above your shoulders.
When the box fits comfortably, measure off 4 pieces of cardboard (use the flaps) from the sheet to the ceiling of the box. Tape one piece to each corner so the sheet stays at the proper level.
Step 4: Making Your Faces
To finish off, add a sheet of mesh behind each face.
Step 5: Wear It!
And that's it! You're done! If you want to, you can also paint the faces and cover in Pixelated coloring to match the Minecraft theme.