The easiest way to get a duvet inside the cover is the following...
Turn the duvet cover inside out....then put your arms inside and pinch the bottom corners with your fingers...then pick up the corners of the duvet or comforter and pinch all the corners together with your fingers.... pull the corners inside the duvet cover by pulling the duvet at the same time you are flipping the duvet cover over your arms and over the duvet!! You will have to tuck in the top corners of the duvet inside the duvet cover at the top...then button it up and you are done!