Add water, sugar and roughly chopped strawberries to a pan.
Boil it until strawberries becomes very soft. Stir occasionally.
Turn off the heat and strain the syrup through a strainer. Press the strawberries to take out all the syrup from it.
Discard the remaining strawberries. Let the syrup cool completely.
Now take a serving glass and add ice cubes to it.
Add chilled soda water till 2/3 of the glass. You can fill till 3/4 as well if you don't like very sweet.
Add the strawberry syrup to fill the glass. Serve chilled.