1. Sew a straight-ish line across the chest of your sweater.
2. Sew the neck hole closed.
3. Stuff the arms like you're turning them into a teddy bear.
4. Insert a pillow into the body of the sweater, and stretch the sweater over it so the front and back of the sweater touch.
It's best if this is a pillow slightly larger than the body of the sweater so that it doesn't slide around too much when you sew it up.