I recently got into growing my own veggies.
Here I have a base of cabbage which I soaked in water after seeing on YouTube. Is that the right way? What should I be seeing and doing next? Should I change water everyday?
You are not really growing cabbage, you are trying to get a bit more edible material from what is basically leftovers. And yes, I know that there are a lot of videos with promising titles á la “never buy veggies again” or something like that.
Problem is, you are working with a severely damaged fraction of a plant, so while you may or may not manage to coax it bit more growth out of them, it’s always going to be just a very temporary hail-Mary attempt by the plant to somehow manage to produce a few seeds.
I suggest you consult your YouTube source and see what care advice they include. Changing the water regularly would be a good idea because you will be fighting mold and rot constantly.