Ensure your mixing bowl is totally clean and dry before adding your egg whites. Whisk these on a high speed for a few minutes, until they become white and frothy. Gradually add your caster sugar a tbsp at a time, whisking for about 15 seconds between each addition. Whisk until it firms up and develops a nice shine. Finally scrape the bottom and sides ensuring it's totally combined and give it one more dig with the whisk.