Is there a fast way to put a duvet cover over a duvet?

Yes! First, lay the duvet out on top of a bed. Next, turn the duvet cover inside out, and place it on top of the duvet, with the bottom facing the top of the bed and duvet. Tie the corners of the duvet cover to the duvet. With a friend, reach inside the duvet cover to grab the furthest corners of the duvet cover with the hand facing the top of the bed, and grab the other corners with the opposite hand. Note: your arms should be crossed. Pull your arms so they are no longer crossed, and stretch them out as far as you can reach. You now have the duvet inside the duvet cover.