Take care when using a mandoline - the blades are super sharp and many people underestimate how easy it is to slice a fingertip when slicing at speed.
You can leave out either the sugar or the salt, but do use one or the other. If you don't have any cider vinegar, white wine vinegar is the next best thing; rice vinegar would work too, but whatever you do, do NOT use distilled or malt vinegar - if that's all you have, just leave the vinegar out.