How do you separate mixture of flour and sugar?

Practicably you can't. However there is a way that will work somewhat. Using a fan with a steady air flow you can sift the mixture of sugar and flour through a wire strainer, so that it falls into the flow of air. The air will carry away most of the flour and the granules of sugar, which are heavier will fall to the floor, or tray you use to catch them. Some flour may fall with the sugar, and some sugar will get carried away, but they will be separated somewhat.