Carefully slice the spaghetti squash in half lengthwise and use a large spoon to scoop out the seeds. Note: If it's really difficult to slice, you can microwave it for a couple of minutes to soften the outside flesh.
Coat the inside of the spaghetti squash with olive oil and sprinkle with salt and pepper.
Turn the spaghetti squash upside down on a baking tray and cook for 35-45 minutes, depending on the size. You'll know it's done when the top of the spaghetti squash can be easily pierced with a knife and the underside edges have become golden.
Let the spaghetti squash cool to the touch, then use a fork to scrape the inside to create long strands of spaghetti squash noodles. Serve immediately.
Notes
Spaghetti squash are notoriously difficult to slice. Make sure your knife is sharpened and take extra care when cutting.