If you want to make perfect cardboard cubes, whether to use as dice, in sculptures, for a hanging mobile or for makeshift building blocks, you can easily craft them in any size you like. All the raw edges will be covered and finished when the cube is complete.
Cutting the Squares
Measure out a square that's the size of cube you want, and use the triangle to make sure your corners are perfectly square.
Put the cardboard on your cutting surface, and lay the metal straightedge over your square to protect it in case the knife slips.
Use the utility knife to carefully cut along the outside of the straightedge, slicing through all three layers of the cardboard.
Repeat this step until you have six cardboard squares of exactly the same size.
Making the Joints
Set aside two squares to be the top and bottom of the cube.
With the other four squares, decide which side will face out on your finished cube, and flip that side down on the cutting surface.
On three sides of the back of each remaining square, measure in 1/8 inch from the edge of the cardboard.
Cut along these lines with the utility knife, but only cut through the first two layers of cardboard -- the back and the corrugated core -- leaving the front side's paper facing intact. These partial cuts will serve as the joints when you put the cube together.
Assembling the Cube
Place one of the uncut squares on your assembly surface, with the good side facing down. This is the bottom of the cube.
Assemble the four sides around the bottom, fitting one joint from each side against the bottom piece, and leaving the uncut edge to face left. This uncut edge will fit into the side cut on the adjacent cube side, and so on.
Once you know how the cube fits together, put a small amount of white glue along the inside of each joint. Piece the sides of the cube tightly around the base again, making sure no raw edges show. Every raw edge should be butted up against the inside of a joint.
Hold each side in place for 10 to 15 seconds to allow the glue to take hold before adding the next side.
When the sides of the cube are assembled around the base, add the top piece inside the four top joints.