Mix the sugar and salt in a glass or small container. Add the water and stir until the sugar and salt have largely dissolved. Then add the vinegar. Then add the oil and stir vigorously.
I always use an electric milk frother to stir the oil or I put all the ingredients in a small, clean jam jar with a screw cap and shake everything vigorously.
It`s a relatively sour dressing. I love it so much, but you can also add more water, then it will be milder. The amount is suitable for half a head of lettuce, rather less.