Prepare a baking pan by greasing it lightly with non-stick cooking spray, butter, or shortening. You could also line the baking pan with aluminum foil instead of greasing it with cooking spray. Use a serrated kitchen knife to cut the squash in half lengthwise. Dig out the seeds and stringy pulp.  Make sure that the squash is clean before using it. The skin of an acorn squash should be scrubbed with a vegetable brush under running water to remove dirt and debris. Use a heavy metal spoon or ice cream scoop to dig out the seeds. Both halves should be cut-side-down in the pan. Do not stack the halves. They can touch, but they must not overlap. Cook the two halves until the flesh feels tender when poked with a fork or paring knife. Do not cover the squash as you cook it. Temporarily remove the squash halves from the oven and brush the cut sides with the melted butter. Sprinkle with salt and pepper, if desired, and return them to oven for another 15 minutes. Note that the squash halves should now be cut-side-up inside the pan. You can enjoy this squash as is, or you can mash it or use it as a base for other recipes requiring pre-cooked acorn squash.

Summary:
Preheat the oven to 400 degrees Fahrenheit (200 degrees Celsius). Cut the acorn squash in half. Transfer the acorn squash to your prepared baking pan. Bake for 45 minutes. Brush the squash with butter and continue baking. Serve as desired.