Extract the juice from the lemons and filter it through a strainer to remove the pulp.

Pour the lemon juice into a large measuring jug, then stir in the caster sugar for 2-3 minutes until dissolved.

Cover the jug and refrigerate for 24 hours, stirring occasionally during that time.

After 24 hours, pour the lemon concentrate into a sterilized bottle and store it in the refrigerator.

Serve by mixing 50mL lemon concentrate with ice cubes and 300mL cold water.