Steep 6 heaped teaspoons of loose mate tea in 1 liter of boiling water for 10 minutes. Drain the tea.
Let the brewed tea cool to room temperature.
Squeeze the 2 medium lemons and add the juice to the cooled tea.
Add sweetener to taste: either 1 teaspoon white stevia powder, or raw cane sugar (for reference, commercial mate sodas contain about 5g sugar per 100ml).
Chill the mate tea concentrate.
Add the 1.5 liters of cold sparkling mineral water to the concentrate and stir. Alternatively, for individual servings, dilute 100ml of concentrate with 150ml cold sparkling mineral water per glass.