First scrub the lemons in warm water, then thinly pare the coloured outer zest from 3 of them using a potato peeler or zester.After that any white pith will need to be pared from the strips of zest with a sharp knife – this is important to prevent the lemonade tasting bitter. Now put the zest in a large bowl and add the squeezed juice of all the lemons (don't bother to strain the juice at this stage) and the sugar. Next pour in 2½ pints (1.4 litres) of boiling water, then stir well, cover and leave overnight in a cool place. Next day stir again and taste to check the sweetness, adding a little more sugar if it needs it.Now strain through a fairly coarse sieve, as it's nice if some of the lemon remains. Pour it into bottles, using sterilised corks, then chill thoroughly. Serve the lemonade either straight or diluted with soda water, with lots of ice.Note: To make lemon barley water, proceed as above but in addition take 4 oz (110 g) of pearl barley and rinse thoroughly under a cold tap. Place it in a saucepan and cover with 2 inches (5 cm) of cold water. Then bring it to the boil and simmer for 3-4 minutes.After that drain it through a sieve and then rinse and drain it again before combining it with the lemon juice, zest, sugar and boiling water. Leave in a cool place for 24 hours. Then strain and bottle as above.You can also watch how to zest lemons in our Cookery School Video on the this page.