Grind plums and sugar in blender. Don’t add any water at this point. Pour out into sieve. Add water into sieve to help drain the purée through the sieve and remove plum skin. Keep adding water as needed till all the plum juice gets out. Fill a jug 3⁄4 of the way up with plum juice and rest with water and ice. Add 2 tablespoons rosewater and stir well. Serve.