Measure the width of the comforter and add 2 inches to the measure. This is the new cover width. Turn the duvet cover inside out. Measure the cover width from hem to hem. Subtract the new width from the cover width. This is the amount you need to remove in width. Divide this amount in two to determine how much to take off from each side.
Measure the comforter length and add 1 inch. This is your new cover length. Measure the length of your cover from the bottom hem to the top edge where it closes. Subtract the new cover length from the original width. This is the amount you need to remove from the bottom edge of the cover.
Measure in on each side to the point you figured for the new cover width. Cut off the excess fabric to make the cover this new width.
Measure from the top down to the point you figured for the new cover length. Cut off the extra fabric from the bottom only to give the fabric the new length.
Pin the 2 halves together and sew around all three cut sides. Use a 1/2-inch seam allowance on all the seams. Turn the duvet cover right side out and insert the queen-size comforter.