Crush the elderberries with a fork (do not use a food processor or you will crush the seeds and it will be bitter). Mix with the vinegar and sugar in a saucepan.
Bring to the boil and cook 10 minutes, until berries are soft. Place in a large glass jar and screw on the lid. Store in a cool dark place for 3 weeks, then strain through a fine sieve lined with cheesecloth. Don't press down the berries too much or the vinegar will be cloudy.
Fill attractive bottles or jars with the vinegar. Store in a dark place.