Place half of the water into a medium-size saucepot. Add the sugar and whisk it together. Place over medium-low heat, stirring constantly until the sugar is dissolved. You know the sugar is dissolved when the mixture becomes clear again. Let it cool slightly on the countertop.
To a large pitcher, add the lemon juice, sugar water, the remaining 2 cups water, and grenadine.
Stir to combine.
Place in the fridge for at least 2 hours to chill and serve over ice.
Notes
You can double this batch if you'd like, see my tips above on the best way to do that.
If you want a less sweet version I would cut the sugar by 1/4 cup.
You can add more or less grenadine depending on how colorful you want your drink.
Fresh squeezed lemon juice is best for this, bottled will not give you the same flavor but you can use bottled in a pinch.
This needs to sit in the refrigerator for at least 2 hours so it can chill and the flavors can blend together.