Place 1 cup of sugar and 1 cup of water in a small saucepan. Bring to a boil and simmer until the sugar is dissolved. Set the syrup aside to cool.
In a large pitcher combine the syrup, lemon juice, and 5 cups of cold water. Taste and add more water (or ice), if desired. Refrigerate until ready to serve.
Option 2 (blender):
Place sugar, lemon juice, and 1 cup of water in a blender. Process briefly until the sugar is dissolved (the mixture will be bubbly/foamy for a few minutes afterwards). Pour this mixture into a pitcher along with the remaining 5 cups of water, stir until combined. Taste and add more water (or ice), if desired. Refrigerate until ready to serve.
Notes
Feel free to adjust the proportions to suit your taste. More lemon juice to make it more tart, more sugar to make it sweeter.Store in the refrigerator for up to 5 days.