1. Chop the squash in half so that you will have two symmetrical halves of equal proportion.
2. Scrape out all the seeds.
3. Place it on a foil covered pan and bake it! Temperature is relative to how long you want to wait for it to cook. I have roasted them anywhere from 350 degrees F to 400 degrees F, for as short as 25 minutes for smaller squash, to as long as an hour for multiple, thicker squashes. The way to know it is done is to pierce it with a fork to check for desired tenderness. You don't want resistance when you pull the fork out. Overbaking them makes for a mushier texture, but they'll be edible either way.
I typically plan on them taking 30 minutes at 400 degrees, that is about how long all four of the halves above took to cook since the acorn squash I had were rather small.
You can bake squash cut-side down, which traps the steam in and prevents them from drying out, or you can bake them cut-side up to allow for filling them or adding toppings (such as DF margarine/oil and brown sugar) at the end of cooking.
In the picture above, I filled the half in the lower left with a pear-applesauce (I dumped in a kids' snack cup and it fit perfectly!) and the one in the upper right had regular sweetened applesauce in it. The two plain ones allowed us to dress them how we liked at the table. Some of us put DF butter on them, some liked DF butter with brown sugar. A sprinkle of Cajun seasoning is also often a nice touch on squash. Acorn squash are mild enough in flavor that they lend themselves well to experimenting with toppings. Butternut squash is also a favorite around here, much sweeter and richer. We have had some buttercup we enjoyed and some we weren't too keen on. Don't be afraid to try new foods, especially when they are healthy!