Fresh filtered apple cider or frozen juice - don't use the bottled juice from the grocery, it has an additive in it to keep it from turning to vinegar.
Raw organic ACV with the mother (unfiltered).
You need to use a glass container! Don't use aluminum or other metal products as it will leach out into the vinegar and ruin it.
Mix 1 qt juice with 1 qt vinegar. Set in a dark warmish place. Check it once a month for the strength you desire.
It is really a good idea to set a big jar of it (gallon) in spring and have it 'incubate' all summer. Then you have awesome vinegar in the fall and winter.
In cold weather it can take up to 6 months for the right mellowness to ripen.