This was a simplified costume we made as a compromise for my girlfriend's Minecraft-obsessed boy, Halloween 2011. It ended up taking the better part of a day to make, but I was very happy with the results. The mask is perfectly (and comically) to scale over small heads, but fits comfortably and attractively on adults as well.
For the head, we used the skin Creeper Mask
as a template. It is made from one piece of 20x30" foamcore cut into 6 10x10" squares and assembled with masking tape after painting (although if you have an extra pair of steady hands to assist your own, regular packing tape would be sturdier and more permanent). Pixels are 1¼" for an 8x8 grid which was measured on all four edges of each square's face, and then lightly drawn with pencil. We cut a crown shaped hole into the sixth piece and taped it into the top of the box at a backward-sloping angle for stabilization and maneuverability.
The sword was also made out of one piece of foamcore, but I cut out two sides, drawing 1¼" grid lines like the mask and painting each color onto both before gluing them back to back with wheatpaste. One side had to be assembled in two separate pieces, reinforced with tape and repainted.