First you need to place your quilt or comforter spread on a large flat surface preferrably the protective bedding on your bed.  If it is a larger comforter it may be helpful to have a helper help out with this task.

Next take your duvet cover and turn it inside out. This will mean that the design or pattern for the exterior will now be on the inside. Now reach inside the inverted duvet cover, find the corners that border the short end of the cover and grasp one corner in each hand. Maintaining your grasp on the corners of the duvet cover, line these up with the same corners that border the short side of the down comforter or quilt. Holding onto the matching corners of quilt or comforter through the duvet cover, start to unroll the duvet cover over the comforter. Feel free to shake the comforter at this point, and if you have a second person they can help. You should now be able to lift the covered comforter and let the duvet cover slide down completely enveloping the comforter.

Lastly, button or zip your duvet cover and lay it on your bed. Most duvet covers are mae so that the buttons are positioned at the foot of the bed. If turned in the correct position you should be able to hide the buttons of the duvet cover on the underside of the cover and comforter.