It's possible to do so without losing any limbs, I promise!
Set the butternut squash firmly on a cutting board and, using a vegetable peeler, peel off bits from top to bottom. I start with the upper part of the squash and the bottom flat on the board, then tilt the squash to carefully peel the bits of peel off of the curved bottom bulbous part. Leave some of the skin on the top and the bottom - that, along with the stem, will help you keep your grip on the slippery squash.
Lay the squash on its side and grip it firmly (but far away from the knife). Lop off the top and the bottom. This gives you a flat surface that will help you keep the squash stable as you proceed.
Place the bottom cut end on the cutting board so it's got nice solid footing, and cut the squash down the middle. You will probably need to press on the back of the knife with the heel of your hand. Slowly, slowly rock and press the knife like a teeter totter to make your way through it until ... voila! Two halves. (If you have an especially large and unwieldy squash, you can cut it in half the other direction too, separating the neck and the body, so that you have four pieces to work with.)
Scoop out the seeds and innards with a spoon and discard.
Cut each half into slices. Cut the slices into spears. And then, a few at a time, cut the spears into cubes. I like about 3/4-inch cubes for roasting. That's it, you're done, and you still have all of your fingers!