Set a large bowl of ice water by the sink. Bring a medium pot of unsalted water to a rapid boil. Stir in the almonds and let cook for 10–15 seconds. Drain the almonds, then immediately plunge them into the ice water to cool and loosen the skins. Remove and rub between your hands to peel away the skins; discard the skins.
Transfer the almonds to a blender with 1½ cups cold water. Blend on high speed until very smooth. Line a fine mesh strainer with 2 layers of cheesecloth and strain the almond milk, pressing the almond pulp to extract as much liquid as possible.
Return the almond pulp to the blender and add an additional 1½ cups cold water. Pulse to rehydrate and combine, then pass the liquid again through the cheesecloth-lined strainer, once again pressing on the pulp.
Spoon the remaining solids into a bowl, and stir in a final 1½ cups of water. Strain though the cheesecloth. Refrigerate the almond milk while you make the rose syrup.
In a small pot, combine the sugar and ⅔ cups water. Set over medium heat and bring to a simmer, stirring to dissolve the sugar. Lower the heat to maintain a simmer and cook until the syrup is slightly reduced, about 10 minutes. Remove the pan from the heat, add the rose water, and transfer the mixture to a heat-resistant bowl. Refrigerate until well-chilled.
To serve, stir the almond milk well, then sweeten with syrup to taste. Serve over ice.