Pierce the spaghetti squash all over with a sharp knife.
Place the squash on a large microwave-safe plate. Microwave it for 5 minutes. Turn it to the other side (carefully; it will be hot) and microwave it for 5 more minutes until it's soft to the touch.
Using oven mitts, remove the squash from the microwave (the squash and the plate will be very hot - use a trivet). Let the squash stand for 5 minutes to finish cooking.
Using a sharp knife, carefully cut the squash in half lengthwise. Allow it to cool for 2-3 minutes until easier to handle.
Scoop out the pulp and seeds from the squash's center.
Rake a fork back and forth to create strands.
Transfer the strands to a bowl.
If desired, toss the strands with butter, salt, pepper, and garlic powder, or use your favorite pasta sauce. Serve immediately.
Video
Notes
If your squash is significantly larger than 2 pounds, start with 6 minutes per side and check to see if it's soft.
Microwave cooking is a bit of a gamble because microwave ovens greatly vary. If you decide that your squash is ready, but upon slicing it, you realize it's still too hard, simply scoop out the pulp and seeds. Place each half in a large, microwave-safe bowl. Add a little water, cover, and microwave until fork-tender. This will probably take 2-3 more minutes for each half.
If the squash ends up overcooked and mushy, place the squash halves, cut side down, on a clean towel for a few minutes. This will help absorb some of the liquid. Take comfort in the fact that it will still taste great!
The leftovers keep well in the fridge, in an airtight container, for up to 4 days. In fact, they taste great even on the fourth day - almost as good as when freshly prepared. Reheat them gently in the microwave, covered, at 50% power.