Wash lemon well. Put ice in large cup (I used a 16 oz cup - make sure you have another one the same size for your shaking!)
Cut lemon in half and squeeze all juice out of lemon into small bowl, discard seeds. You can also get the lemon pulp out using a fork, and then pour lemon juice and pulp into ice.
Add water, sugar, and what's left of your lemon, top with other cup and SHAKE! Add any additional sugar to taste. Pour into pretty glass and serve.