Put berries in blender and process; strain for smooth puree.*.
Make simple syrup: combine sugar and water in small saucepan. Heat over low heat, and stir until sugar is completely dissolved. Increase heat to medium-high and bring to boil. Reduce heat to low, and cook 3-5 minutes. Cool.
Add 1/2 cup of syrup. Stir in juices; add soda to taste.
Rest of syrup can stay in refrigerator, in very clean glass jar, for two weeks. Can be used as you would other sweeteners.
* using 2 cups diced strawberries would be a good starting point.