While you can do it that way, I don't know the process. I know how to make tannin by soaking oak logs in water.

And yes, that is probably the first way we figured out how to do it, but as I understand it, that's a very stinky process, and to be totally honest, I found tannin made from oak to be rather smelly.

In traditional tanning different chemicals are used to process leather. Brains, egg yolks, urine, they all work. The urine isn't a requirement, it's a possible chemical compound a tanner may choose to utilize.