Add 1 heaped teaspoon of tea leaves into a teapot and steep in boiling water for 5 minutes. You want a strong concentrated brew, but less concentrated than for making tea ice cubes.
Strain the tea and use an electric frother to froth it up a bit.
Pour the tea over chilled tonic.
Notes
The easiest way to make a strong brew with 50 ml of water is to use a Chinese gaiwan or a paper tea filter.