From my sister:
If you can't get the gallon tea bags, use regular Lipton (so orange pekoe) bags. She thinks you'd need about 8 tea bags, but you can adjust this as you experiment. Get a big saucepan and fill it with about a third of a gallon of water. Put the teabags in it and bring it to a boil. While it's coming to a boil, take a gallon jug put 3-4 cups of sugar (note from me: OMIGAWD) in it. Once the tea is boiled, take a spoon and hold the teabags back as you pour the tea into the pitcher with the sugar in it. Stir it up to dissolve the sugar. Add ice (you can let it sit and cool next time if it gets cloudy, but my sister swears by the ice at this step). Fill the rest of the pitcher with water. You can multiply this or divide it as needed. She usually makes more than one gallon at a time.