I vary it based on if the cover came out the machine inside out or not.
If it did, I spread out the duvet, burrow inside the cover, grab the far corners of the cover from the inside and then through it also the corners of the duvet at the same time, and invert it.
If it came out the right way round, I spread out the cover, grab two corners of the duvet, burrow inside the cover with the two corners of the duvet until they reach their destination, and come out again.
If it did, I spread out the duvet, burrow inside the cover, grab the far corners of the cover from the inside and then through it also the corners of the duvet at the same time, and invert it.
If it came out the right way round, I spread out the cover, grab two corners of the duvet, burrow inside the cover with the two corners of the duvet until they reach their destination, and come out again.