It feels rather James Bond this drink! Not sure why - using the cocktail shaker most likely. If you don't have one of those just use a tall container with a lid.
2 cups ice
Juice of 3 lemons
1/2 tsp orange or lemon essence
1 tsp caster sugar (optional)
Put the fresh lemon juice, essence and sugar (if using) into a cocktail shaker filled with ice. Shake well.
Pour this mixture into 4 champagne flutes.
Top up each flute with tonic water.