5-7 ounces (depending on taste and the sweetness of the berries) sugar
Juice from 1 lemon
Carbonated water
In a medium saucepan, bring the water and the berries to a boil. Turn the heat to low, and simmer for 20 minutes, stirring occasionally.
Line a colander with a couple layers of cheesecloth, and set it in a large bowl, and set that in the sink. Carefully pour the berries into the strainer. Allow them to sit there to drain and cool for another 15 minutes, at least. When the berries are cool enough to handle, pick up the sides of the cheesecloth, and form a bag around the berries. Squeeze out as much juice into the bowl as you can. Discard the pulp and peels that remain in the cloth.
Return the blueberry juice to the saucepan. On medium heat, stir in the sugar and lemon juice. Allow this to reduce for at least 10 minutes. This gives you a syrup from which you can make your soda.
Pour it into a heatproof container, and let it cool. When you're ready to drink it, just mix a little bit of the syrup in a glass with the carbonated water. Serve over ice.