I don't do anything - I just have a regular filter but my understanding is that the people who do this do certain black magic which somewhat depends on the "hardness/softness" and ph of the water as well as other stuff but it's all in that book apparently. I don't know the details.
I went down the rabbit hole trying to learn about this, and it’s actually pretty easy to replicate the recommended water. Home brewing (beer) water is far more complex than this.
I’m going to brew some with this profile and see if I can taste the change, and if not I’m just sticking with my filtered tap water because it’s honestly pretty darn close. Just needs the chloramine taken out.
Edit to add: Actually I just found this resource which you might find helpful https://dailycoffeenews.com/2018/08/15/a-practical-water-gui...