Bring 8 cups of water to a boil, and place 8 to 10 heaping tablespoons of your favorite locally bought black tea leaves into a large pitcher or pot. Pour boiling water over the tea and steep for 5 minutes.
Strain the tea into a second pitcher (this removes most of the loose tea), then use a coffee filter to strain it back into the (rinsed) first pitcher.
Add sugar to taste (I recommend at least 1 cup, as Thai iced tea should be very sweet), stir and let cool. Place tea in the fridge.
Pour chilled tea over ice (preferably crushed ice) and top with evaporated milk. Let the milk float on the ice and slowly swirl into the tea.