Fold your duvet cover neatly into four (i.e. into two and then into two again). Shake your duvet cover a couple of times after each fold to help the fabric fall neatly in the right places. Iron the two outer sides, pushing a little harder and moving a little more slowly than normal on the fabric (because the heat has to go through more layers). Then, open up the second fold and place the freshly ironed outer sides on the inside so that you can now iron the inner sides. This should give satisfactory results.
However, if you want your duvet cover to have a more “ironed look”, open up the two folds and fold the duvet cover into four again, placing all inner sides on the outside and repeat the steps shown in the previous paragraph. I hope this tip on ironing makes sense!