In a large bowl, combine the masa harina with 2 cups of the cold water. Knead with your hand until a smooth, cohesive dough forms and no dry flour remains, 2–3 minutes.
Gradually whisk in the remaining 4 cups cold water until the masa dissolves and the liquid is evenly milky with no lumps.
Let the mixture rest 10 minutes to fully hydrate, then whisk again to re-suspend any settled solids.
For a smoother texture, strain through a medium‑mesh sieve into a pitcher, pressing gently with a spoon; whisk once more to combine.
Serve in cups over ice cubes if desired. Stir before pouring and between sips, as the masa naturally settles.