A kombucha 'Scoby' is a Symbiotic Culture of Bacteria and Yeasts which is used to ferment a solution of sugary black tea.
Make a brew of 4 black tea bags with 1 cup of sugar, (white, brown or raw) in about 2 cups of hot water. Stir to dissolve the sugar. Pour into a large 2 - 3 litre glass jar and add good quality water to almost the top of the jar. (not from the tap) Cover with a lid lightly screwed down. Sit it on your bench out of direct sunlight and try not to move it for at least 1 week.
After that time, taste a little of the liquid. If it's ready, it shouldn't be too sweet, nor should it be too vinegary. Remove the 'scoby' to a dish with a little of the liquid. Decant the rest of the liquid by pouring through a sieve resting on a funnel, into 2-3 tall bottles with a wire clip lid. At this stage other flavourings can be added for a second fermentation to give different tastes each time. For example cinnamon bark, cloves, allspice, or fruit pieces like apricots, figs, apples, ginger - it's up to your imagination what you put in it.
Leave the bottles at room temperature for 1-2 days then place in the fridge for storage. You may need to burp the lid once or twice over a week to release the build up of carbon dioxide.