Using a two litre fizzy drinks bottle (don't use glass, it is way too exciting when things start happening and then booofff, glass everywhere)
Ring the bottle and then rinse again with some light sterilizing fluid, Milton's fluid or maybe a part of a Camden table dissolved and swilled around.
Shred around an ounce of fresh ginger and add to the bottle.
Juice half a lemon and add to the bottle.
Add a half teaspoon of wine yeast (another recipe says any yeast but I cant confirm)
Add a cup of sugar
Half fill the bottle with warm water and give it a serious shake.
Top up with water and store somewhere warmish for at least 48 hours
Leave it longer for a less sweet, more alcoholic ginger beer.
If you want to keep it longer but keep it sweet/lower on alcohol then add a dissolved Camden tablet (not sure I fancy that)
Oh, and release the extra gas most days or there will be explosions in your larder