I was a kid, but I've never done myself. I watched my dad many times, and it always worked to the best of my recollection. Here's basically the process:

1. Try to get a 2-4 volt charge into the battery. You may have to add water and set the battery on a charger for 24 hours or so. All of my battery charges are smart chargers with a chip that kick off and won't charge a completely dead battery, so to get a completely flat battery to start charging, I take a working battery (usually my trolling motor battery) and connect it to the dead battery with jumper cables. Then I put the charger on the good battery. You may have to use this process to get some charge in a battery you're reconditioning.

2. After you get some charge into the battery, pop off the caps and dump all the acid out. Then mix about a cup of baking soda with a gallon of warm tap water. Then add the baking soda/water mix to your empty cells. Be careful. The more charge you had in your battery, the more violently the reaction and bubbling will be when you add this. Let this solution sit in your battery until it stops bubbling (maybe an hour). The bubbling is breaking down the sulfate crystals on your lead plates. Then dump the baking soda water out.

3. At this point you need to refill the cells. If you can get 65/35 acid, this is best. You have to order it online because auto parts stores stopped selling it. If you don't have any actual battery acid, you can mix a gallon of distilled water with a cup of epson salt and then heat it until the salt completly dissolves. Then fill the cells with the epson salt/distilled water. It's not necessary, but at this point is where my dad always added the Charge-It to the cells to help build the acid solution. Then charge your battery with the caps off at a trickle charge for a day or two.