You need my (patented) banister method of putting on a duvet cover, katek
Wash duvet cover inside out and dry it and put it away like that too.
Hang duvet over banisters on landing.
Put your hands (and arms - but try to keep your head out!) inside the duvet cover (still inside out) down into the two corners.
Grab the two top corners of the duvet with each hand, through the duvet cover.
Drop the duvet over the stairwell, with the cover falling over the outside of it. Shake it well (don't let go!)
Drag it back over the banister and into the bedroom and onto the bed. If there are two of you, you can grab a corner each and each walk up one side of the bed with it held between you. If there is only one, you have to do the best you can here.
Straighten it up, get the bottom corners in, and fasten it at the bottom.