Brew the mate tea with 1 liter of boiling water and let it steep for 10 minutes. Drain and let the tea cool down.
Squeeze the lemons and add the juice to the tea. Now sweeten the tea as you like.
I use 1 level teaspoon of white stevia powder for this.
Those who prefer to sweeten the classic way with sugar can orientate themselves here: A well-known, commercially available soda based on mate tea contains 5g of sugar per 100 ml. That would be 125g for 2.5 liters of soda.
Chill the finished mate tea concentrate and add a total of 1.5 liters of cold mineral water. If you don`t want to drink the 2.5 l soda at once, it makes more sense to dilute the soda by the glass (e.g. 100ml mate tea concentrate + 150 ml mineral water).