Fill and fit
A duvet cover is a case for something you already own. Getting the pairing right is most of whether it looks made in the morning.
Every duvet cover in this catalogue — 374 of them — arrives without filling. That is the point of the form: you keep one insert and change the picture as often as you like, and the cover washes as easily as a sheet.
Match the insert, do not round up
Buy the cover in the same size as the insert. A cover cut larger gives the filling room to slide, and it will: within a week the loft has migrated into two corners and the foot of the bed is flat. If your insert falls between sizes, go to the smaller cover, not the larger one.
Corner ties earn their keep
The loops sewn inside each corner exist to be tied to the insert's own tabs. It takes two minutes once and stops the slide permanently. If your insert has no tabs, a few hand stitches of matching thread do the same job and survive the wash.
Getting it on without a fight
Turn the cover inside out, lay it on the bed, put the insert on top, tie the two top corners, then roll the pair from the head of the bed towards the foot like a sleeping bag. Reach in, grab the rolled bundle through the opening, and unroll. It is the one trick worth knowing in this whole category.