Ok so this is one thing I never have trouble with. My trick is;
1) turn the duvet cover inside out
2) grab the bottom corners of the duvet and then hold the corners of the duvet cover (the top / unbuttoned side)
3) flip it so the cover falls over the duvet
4) then grab the other end and put the corners in and do it up
You might need to give it a good shake a few times to get rid of bumps but it works like a charm.
Hubby can never understand how I can do it so quick, he takes about 10 min to change the bedding and lands up crawling inside the cover to try sort out the corners. :lol: