Put first measure of sugar in warm water to dissolve, add yeast and stir. Place in warm place to start working.
Finely grate or slice rind from 2 lemons and place in a heatproof container with the 1 cup of sugar and the dried ginger. Pour over 1 cup of boiling water and leave to steep for 10-15 minutes. Strain into 1.5 L plastic bottle in which the ginger beer will be made. Top up bottle with cool water to near top so that final temp is approx. body temp. Add yeast to bottle as soon as it foams. Cap bottle tightly. Shake thoroughly and put in a warm place. Leave it until the bottle becomes undentable. Depending on the yeast this can take anything from 12 hours to 24 hours, but best to check regularly, as there is a risk of explosion with this! Refrigerate until thoroughly chilled, open with care and serve.