Simply leave an open, 3/4 filled bottle of wine in a warm place for a couple of weeks..ta da!
This technique will give you just one bottle of vinegar. If you are feeling a little more industrious than that then take a wide-mouthed glass jug with a capacity of at least a gallon. Pour in a couple of pints of wine and a cup of vinegar to liven things up.
Keep the container covered most of the time, but open it for a half hour every day to let it breathe.
In a couple of weeks the madre, a viscous starter, will have settled to the bottom of the jug, while the vinegar above it will be ready for use. Add more wine as you remove vinegar to keep the level in the jug constant and you will have a never ending supply.
NOTE: Please do not use any of our wines in such a manor, but if you can’t finish a bottle then why not give it a go?