It's actually pretty simple. 3 tbsp salt (Kosher or sea salt. I've used both and can't tell any difference in the end product) per 5 lbs of cabbage.
Shred or thinly slice the cabbage, saving a few untorn outer leaves.
I do a couple of heads at a time (or whatever can fit into the giant metal dough mixing bowl I have), sprinkle salt on the cabbage and squeeze it a bit to get the juices flowing. Once I can squeeze it and it wrings out like a damp towel, I start packing it into the crock (or in my case, glass cookie jar). Pack it as tightly as you can. I've tried various "packing" implements and have found that my fist does the best job. Do that with the next batch of cabbage and so on and so forth. You may want to use gloves when you're doing this. My knuckles would be red and raw afterward and stung like heck. The salt did no favors to my skin. You also discover cuts you didn't know you had.
It's amazing how much cabbage you can pack in. Keep packing it down and you'll see the water level rise as you mash and the salt draws it out. Some people add a brine to it to raise the water level, but I've found it unnecessary. It all draws out eventually. When that's done, I turn the leaves I put aside inside-out and lay them on the cabbage to keep stray shreds from floating up and cover with a plate. A weight of some sort is necessary to keep everything under the water level. I use a 2 gallon zip lock freezer bag with a brine (1 tsp salt per 1 cup water) in it to weigh everything down and as my airlock. The brine is in case the bag leaks. Put the bag into the jar and fill with water until it seals around the rim.
Do whatever you need to the lid to form an airtight seal where CO2 can escape but no Oxygen can get in. My next adventure (going for 25 lbs), I'm using a fermenting bucket with a proper airlock on it. It will make life so much easier.
Next, put your infant kraut away in a dark place. Temperature wise, the room I ferment it in stays about 60-65F. Some people have said that was too high while others said it was too low. It works just fine for me. Check the kraut daily. With a paper towel, skim around the inside walls if there's any white scum. Pack it back down if needed. Taste the kraut. It is ready when you like it. Mine's ususally ready when I think it went bad. I like it somewhat crisp with a nice sour bite.
I pack mine into zip locks with just enough juice to cover the kraut. A few bags stay in the fridge while the others go in the freezer.