Steep 5 black tea bags and 1 mint tea bag in 4 cups of water in fridge. After 24 hours, remove mint tea bag and leave black tea bags for another 24 hours before removing those, too.
Make lemon simple syrup (1 cup water, 1 cup sugar, juice of 1/2 a lemon). Stir desired amount into iced tea to sweeten.
Note: You could use fresh mint for this as well, but I don't prefer it because I personally find it overpowering.