Slice heads of cabbage and carrots (using knife or mandoline). Place everything in a large bowl and mix well (work in batches as needed). Add salt (2 TBSP if you like it less salty, 3 TBSP for more salty) and mix through, squeezing to release water. Place everything, including water, in a crock and weight down with a plate or bowl. Everything should be under water - if it's not, add a little filtered water. Cover with cheesecloth or a coffee filter and sit on top of the fridge for 2 or more weeks, or until taste is to your liking. Check every few days to make sure water level stays above cabbage. If mold happens to begin to form, remove all of it immediately.