Putting on a duvet can be tricky as the cover itself is large and so is the comforter that you are trying to get into the duvet. Here's my trick for easily getting a comforter into a duvet.

To start, lay the comforter out flat on your bed. Next, turn the duvet inside out. Then put your hands into the inside of the duvet and grab the back corners, one in each hand. With each hand, then also grab a corner of the comforter. Pick up the entire assembly and shake so that as the comforter is coming up, the duvet will fall down and turn right-side out as it falls. Once you pick up the whole assembly, the comforter should be in your duvet with the corner nicely in place.