fresh, green olives (like manzanilla or sevillano)
apple cider vinegar (or other lightly flavored vinegar)
Place the unwashed olives in a large vessel such as a crock, glass jar or food-safe plastic bucket. Discard any olives that are bruised or misshapen. Make a brine using the following formula: for every 2 quarts of water, add 1 cup vinegar and 3/4 cup salt. Make enough brine to cover the olives completely by at least 2 inches. Weight the olives to keep them all submerged in the brine. Cover loosely and store in a cool place, out of direct sunlight, to ferment for 3 months. Skim any scum that forms on the surface as needed.
After 3 months, drain the olives, but don’t rinse them. Create a new brine and let the olives sit for another two months, then begin to taste weekly. When the level of bitterness has reduced to your liking, drain the brine and make a weaker storage brine: for every half-gallon of water, add 1/2 cup salt and 3/4 cup vinegar. You may also add any flavoring agents like garlic, herbs, etc.
Store the olives in their new brine in the refrigerator and enjoy. Kept covered and cold, these olives will last at least six months. If you find the olives too salty, let them soak in clean water for 4-8 hours before eating.