Peel it deeply. Celeriac is knobbly and roots grow out of the surface, so a peeler skates over the top of it. Take a centimetre off with a knife and cut away anything hollow or brown.
Dress it as soon as it is grated. Cut celeriac and apple both brown quickly, and the lemon juice in the dressing is what stops it.
Under-season at first. Salt keeps drawing water out while the salad chills, so taste and season again just before it goes to the table.
The half hour in the fridge is not optional. Cold firms the shreds up as well as softening the raw edge of the celeriac. An hour is better.
Eat it within a day or two. It is at its best on the day and the dressing loosens as it sits, so give it a stir before serving.