Brunwater is available from Martin's website:
https://www.brunwater.com/
To get you going I downloaded the free version and have attached it to this post. I've also filled it in with the data from last brew to show you how to use it. Here's what I did.
1. On the pale blue "1. Water Report Input" tab I entered the mineral values for the water I'm using (Tesco Ashbeck) at the top under "Water Report Input" in the pale blue cells (B6, B7, B8, C6, C7, C8, C9). Ignore everything else on the page.
View attachment 41000
2. On the orange "2. Sparge Acidification" sheet I entered 6.2 as the pH of Tesco Ashbeck and 5.3 as the pH of the water I want for my brewing (B5, B6).
View attachment 41001
3. On the Yellow "3. Grain Bill Input" I entered the grains in the recipe. I fill in the "Grains", "Quantity (g)", "Color (EBC)" columns and ignore the "Quantity (kg)" column.
View attachment 41004
4. This is the big one. On the Red "4. Water Adjustment" page I enter the mash water volume (J17), sparge water volume (L17) and the total batch volume into the fermenter (N17). Under the "Water Additions" row 17 I play with the Gypsum, Calcium Chloride, Epsom Salts values (B19, B20, B21) until the "Mashing Water Profile" row 13 looks like I want it to. Now I play with cell B29 (lactic acid) until The Estimated Mash pH (B16) comes in at 5.30.
View attachment 41005
5. Finally I print off a copy of the White "5. Adjustment Summary" page because that's what I'll need to add on brew day.
If you need help let me know.