Roll the lemons on the countertop to loosen the juice, then cut in half and juice them using a citrus juicer. If you have any pull or seeds, run the juice through a strainer. This should yield about 1 ¼ cups of fresh lemon juice.
In a microwavable container, heat 1 cup of water with 1 cup of sugar until the sugar dissolves. Mix really well.
Pour the fresh lemon juice and the sugar syrup into the pitcher. Stir well to combine.
Add the remaining 3 cups of cold water to the pitcher and stir. Taste and adjust the sweetness or tartness by adding more sugar or water if needed.
Fill glasses with ice cubes. Pour the lemonade over the ice, and garnish with a lemon slice.