Scrub the lemons clean under warm running water, then cut off about 1/4 of an inch off the tip of each lemon.
Make a deep cross in each lemon, as if you were going to cut them into quarters lengthwise, but don't cut them all the way through. You want to stop about 1/2 an inch from the end so to keep the lemons attached at their base.
Pry the lemons open with your fingers and sprinkle about a tablespoon of salt all over the insides of the lemons.
Add the the lemons to the jar, one at a time, and squish them down real good so that they release enough juice for it to eventually rise all the way to the top of the jar.
Make sure the lemons are completely covered with lemon juice. Add more freshly squeezed lemon juice (or filtered water if no spare lemons are available) if necessary.
To help keep the lemons completely submerged, you may want to add a little bit of weight on top of them, such as a shot glass or any other small non-reactive object.
Cover the jar loosely and let it ferment at room temperature for 10-12 days then transfer to refrigerator.
If you're using a latch lid jar, you can either remove the rubber ring during fermentation or "burp" it every day by slightly cracking open that lid to allow excess gas to escape.