Place the squash on a cutting board that doesn't slip, and use a sharp knife to slice off the stem and bottom.
Cut the squash in half crosswise at the base of the neck, right before the bottom curves out.
Cut the bottom of the squash in half lengthwise, and remove the seeds with a spoon.
Slice the top of the squash in half lengthwise, then cut each half crosswise into 1-inch strips.
Stack a couple of the strips in top of each other, and cut them in half.
Dice the strips into 1-inch cubes. Repeat with the rest of the squash.
Video
Notes
If you want to soften the squash before cutting it, make several small slits in the skin of the squash (this is very important, so the squash can release steam in the microwave). Then, place it in the microwave and cook for 2 to 3 minutes on high to soften.
The squash will be hot when you take it out of the microwave, so use a towel of oven mitts to handle it.