I got this recipe from a Martha Stewart cookbook, it's delicious!
1 cup sugar
1 cup fresh lemon juice (from about 7 lemons)
1 pound strawberries
16 oz seltzer
In pan, bring 2 cups water and sugar to boil over medium-high heat. Reduce to a simmer and cook, stirring occasionally, until sugar dissolves, 2 minutes. Transfer to a pitcher and refrigerate until cool, about 45 minutes. Add lemon juice and stir to combine.
In a blender, puree strawberries until smooth. Pour through a strainer. I didn't do this cause I like the fruit bits. Stir in seltzer.