[spuz]

This is what I do. I take a corner of the inner and stuff it into the outer until I find the corner of the outer. Then I try to keep those two corners in place while I do the same with the other corner. Then I grab both corners from the outside and do a lot of vigourous shaking until everything lines up. It takes ages and doesn't always work. I think I will try starting inside out from now on.

[zelphirkalt]

IME neither inside out nor the method you describe work well and both are tedious. Inside out does not work well, because the cover doesn't obey gravity and refuses to fall down to cover the duvet completely. It is a secret power of bed sheet covers. That, or it has to do with other things like friction.

[vladvasiliu]

I do a modified version: I put all the corners in the right places, then a good shake by holding two adjacent corners straightens everything out. May not work so well for duvets much wider than your arm span.

[Jare]

I do that and while the shaking is unpredictable and often requires doing it from multiple sides, I find it a strong but strangely pleasant exercise for my shoulders.