My husband doesn’t like duvets so we have sheets and blankets on our king-size bed.
Although it is a tedious task I iron the sheets. When I iron the top sheet I fold in in half, then half again lengthwise. The sheet now fits on my standard size ironing board. I iron the top quarter then flip it over and iron the back.
Next I refold so that the inside becomes the outside and repeat.
Avoid pressing too close to the folds as this will form a crease.