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What is your starting water? Are you using deionized water or tap water? If tap water, what is the source?
Starting off, you need to understand the base water you are starting with. I would recommend highly reading Martin Brungard's publication on brewing water which can be found at https://www.brunwater.com/water-knowledge as a starting point. From there, ask any clarifying questions you may have.
I do not pretend to know anything at all about water chemistry so I rely completely on the results Beersmith gives me. To start with I use my tap water (well water). I sent a sample to Ward Labs and ordered their Brewers Test Kit. It returns to you only what you need to know to set up your starting water. https://www.wardlab.com/product/water-test-kit/
When you get your results back use the information to build your Base Water Profile. Now you are ready to start. In your recipe click the water tab and you will first select your Base Water Profile. Next you will select a Target Profile. This is the water you are trying to match. Beersmith will compare the target you are trying to mimic with your base water and tell you what you need to add and how much of it.