Oh! I was taught an easy way to do that, which I shall share whether you are interested or not.
Turn the duvet cover inside-out, and put your arms into it down to the two far corners from the opening.
Take hold of two of the corners of the quilt (ones that are the right distance apart to match the corners of the cover that you are holding) through the cover, so that you are holding both inside-out cover corners and the corners of the quilt.
Slowly ease the cover over the quilt, turning it right way out as you go and keeping a good grip on the corners.
When the cover is more or less over all the quilt, let go of the corners you were holding, take the two corners by the opening by both the cover and the quilt, and shake it to get it all matching.
Do up the poppers or buttons or zip of the cover.
Done.
Takes me a lot less time than other methods used to.
Since my friend told me this (she got it off a television show about families doing household tasks, I think) I have never ended up head-first inside a duvet cover trying to find two corners inside a welter of cloth. It was a game-changer!