First off, use a large and sharp chef's knife to cut the squash in half, and make sure that it's stable on a chopping board before you cut it. You can cut following down the natural grooves of the squash which can also prevent slipping. Use a firm and hard downward pressure on the knife, don't saw it back and forth. Keep your fingers clear and go steady! Once cut in half, use a spoon to scoop out the seeds and fibers, then lay the cut side down and cut into slices.