Flip the duvet cover inside out. Doing this will allow for easier application on the actual duvet.
Place your hands inside the duvet cover and grab two of the ends. According to bbc.co.uk, the top corners will be the best to hold. Lift the cover and the duvet by holding on to the bottom ends of the duvet.
Shake the duvet cover and the duvet to allow for the comforter to slip into the cover. While doing this, the cover will begin to change to its original form and it will not be inside out anymore. Bbc.co.uk says that the duvet should fall down into the duvet cover. You can help it move along by slowly pushing the duvet into the duvet cover.
Check that the comforter is completely in the duvet cover. Make sure the duvet and the cover are smooth and bump-free. Smooth out bumps with your hands.