Pour apple cider into a very large, non-reactive stockpot (stainless steel, copper, or glass, but NOT aluminum unless it's coated.)

Use a clean ruler or wooden stick that you can mark with the starting level of the cider.

Turn heat to high, cover the pot with a splatter screen (to prevent flies or other insects from dropping into the pot) and bring to a boil.

Boil the cider hard until it has reduced to 1/7th of its original volume. Watch more carefully toward the end because it may creep up higher in the pan as it becomes thicker and bubbles stack up on each other. Turn off the heat and let the bubbles die down to check the depth of the liquid with your ruler or dipstick. You should end up with approximately the volume that it takes to fill a clean, empty 750ml wine bottle.

Pour into a clean, sterile jar (for long term storage) or a clean, empty wine bottle (for short term, refrigerated storage). Cover tightly and store in a cool, dark place.