Start by thoroughly washing and drying the squash. Next, slice off the ends of the squash, removing about ½ an inch from both ends. This gives you a flat surface with which to work.
Next place the squash with the flat end down. Slice from the stem end through the tip end. You may need to use a sawing motion, or use your other hand as a helper to press down and slice the squash in half.Use a metal spoon to remove the seeds and the membrane. You can either compost the seeds or save them to roast.Now you'll have a vessel if you want to stuff your squash.
If you're planning to cut your acorn squash into slices instead, turn the scooped-out half face down and cut cross-wise. Make your slices about ¾ inch thick. Try to cut them all the same width so they'll cook at the same rate.
To cut wedges or "boats," place the squash halves flesh side down and cut lengthwise along the ridges, usually every second one.
And finally, if you want to cut an acorn squash into cubes, cut lengthwise along each ridge, then cut each wedge crosswise into cubes.
Notes
There's no need to peel acorn squash. The skin is edible and it becomes soft once it has been baked. If you don't want to eat it, it's easy to slip it off.