To cut the neck off a sweatshirt, you simply need to cut along the ribbing and plan your new style. There is no need to sew the new edge if you don't want to, because it is no likely to fray.
- Cut along the ribbing
Turn the sweatshirt inside out and use sharp scissors to cut just inside the stitched seam of the neckline. If you do not desire a different style, you're done.
- Mark your new neckline
Put the sweatshirt on and mark the desired neckline with a fabric marker. A popular option is a v-neck.
- Cut Your new neckline
Remove the sweatshirt and fold it horizontally. Cut along your marks, making sure to cut through both sides of the sweatshirt so that you are left with a balanced look. If any fabric marker line remains, wash before wearing.