This age-old problem arises in most duvet-centric cultures. My own research with the doona ( the Antipodean relative of the duvet) has shown the above method to be highly dependent on the coefficient of friction between the cover and the insides.

If this is too high, it is likely that steps 3 and 4 will result in bunched up insides inside a bunched up cover - a state which will only become more intractable with any efforts to amend the situation.

with this in mind I proffer the following steps to replace steps 3 and 4 above to allow for this case

3. Turn the cover inside out.
3a. Get inside the cover. no really
3b. Hold you arms above your head and manoeuvre your hands into the corners of the cover
3c. Realise you cant see anything in here. Get out, place the insides within easy reach and memorise where they are. Get back in and repeat previous steps.
3d. Grab one corner of the insides in each hand through the cover. Hold on tightly.
4. Bend forward at the hips and shuck, writhe and generally shake your groove thang until the cover is rightside out again. Do NOT let go of the corners.

you should now be able to continue at step 6. Turn off the ceiling fan first.