Dry roast cumin seeds in a non-stick pan till fragrant.
Quarter the lemons and place in a glass jar. Add castor sugar and black salt. Roughly tear mint leaves and add.
Crush roasted cumin seeds in a mortar with a pestle. Add this to the jar and muddle with a muddler.
Add lemon juice and orange squash and continue to muddle.
Place some ice cubes in 4 stemmed glasses and pour the prepared juice over. Top with soda and mix well. Garnish each glass with an orange slice and serve chilled.