I'm terrible at putting duvets in duvet covers. Regardless of the technique I try it always ends up with me having to go inside the duvet cover with the duvet to fix it up. Maybe it's time to admit defeat and ban duvets at my house.
@Kimeros
I've found success with the inside out method. Turn the cover inside out, reach your hands into it, all the way in to the far corners. Use those corners like gloves as you pick up the corners of the duvet inner, now shake like crazy and the cover is on the inner and just needs to be pulled down to reach the edges. At least that way the trickier corners are already done and you just need to straighten it up :)