2 lemons, 100g sugar, 500ml boiling water.
Thinly peel the lemons. I use a potato peeler. Put the peel in a jug with 100g sugar (the original recipe was 4 oz sugar, but this is too sweet and 3 oz is too tart, I think), add the boiling water. Stir, cover and leave to cool. Squeeze the lemons, add the juice and pulp, strain into a bottle and refrigerate. Dilute as much as you like.
A splash added to iced (milkless, of course) tea is lovely too.