Place the squash on a kitchen towel (to prevent slipping) on a hard surface. Stab the squash 5 - 6 times with a sharp knife, making sure to pierce the exterior flesh and down into the cavity of the squash
Microwave the squash for 4 minutes to soften it. Allow it to cool off in the microwave until it is cool enough to handle.
Carefully cut the squash across the width so that you now have two pieces of squash.
Place one half of the squash into a microwave-safe dish and pour water into the dish until it covers about 1" of the base of the squash.
Microwave for 12 - 18 minutes, based on the size of the squash. Smaller squash should be done after 12 minutes, and very large squash may take up to 18 if not 20. Squash is ready when the flesh starts to look soft, slightly darkens, and the interior can be easily scraped into shreds.
There are tons of great recipes for spaghetti squash, but my favorite way to eat it is exactly as written above, with a pat of butter and some fresh cracked pepper on top.