Cut off the lid of the grapefruit, hollow out the fruit and fill 3/4 of the way with yerba mate. Cover the opening of the grapefruit with the palm of your hand and shake gently to remove the dust from the mate tea. Hold the grapefruit so that it forms a slope with the tea. Put the drinking straw into the grapefruit and add ice cubes.
Mix the lemon juice, ice cubes and water in another container. Pour enough lemonade into the grapefruit to cover the mate tea.