Tackle the farthest corner first, crawling on hands and knees and engaging your core muscles. Ease back to fit other corners.
For changing pillowcases: Place pillow under one arm, gather the case together, and fit it over the end, shaking the pillow in. (Don’t pin the pillow to your chest with your chin. That puts the cervical spine in a hyperflexed—i.e., bad—position.)