Basic Lassi

Equal parts:
Ice cold water
Yogurt

To Taste:
Lemon juice
Rosewater
Sugar
Salt

Stir together yogurt and ice water - the colder the water, the better the consistency.  Add rosewater or lemon juice to suit your taste, then sugar or salt - I personally never add sugar, but instead use vanilla yogurt or Greek yogurt with honey.  And I like my lassi sweet rather than tart so I never add lemon juice or salt.