Put egg whites into a large mixing bowl and whisk lightly with a fork until bubbles begin to form on the surface.
Add about half the icing sugar and the lemon juice, and beat well with a wooden spoon for about 10 minutes until brilliant white. Gradually stir in the rest of the icing sugar until the consistency is correct for piping.
Once made, keep the icing covered with a damp cloth to prevent it drying out and use as soon as possible.
