This could hardly be simpler:
Take an empty water bottle (the 300ml or 250 ml size; it doesn't matter which) and fill it with cold water. Add 1 spoonful of tea leaves, or, more conveniently, one teabag. Ordinary tea is fine, but more delicate teas (jasmine, white tea, green tea, darjeeling) are very good too.
Leave in the fridge overnight.
That's it! It's about the only non-alcohlic drink I want in this very, very hot weather.
You can add a sliver of fresh lemon or lime, and a couple of ice cubes if you think that it ought to involve more work.