Step 1: Gather Your Supplies
You will need:
plastic cup or weigh boat
5 gallon cornelius "Corney" keg w/ fittings
co2 tank and regulator
Step 2: Prep Keg
Clean out keg. This is especially important if had previously been used to store soda or beer. Use starsan and be sure to clean the o-rings
Step 3: Fill and Dechlorinate
Fill keg and let sit overnight with top opened. This should let the chlorine dissipate.
Step 4: Carbonate Water
This forms carbonic acid which helps the minerals to dissolve
Step 5: Add Minerals
Place cup or boat on your scale and tare. Add 5.60 grams of magnesium hydroxide OR 8.10 grams of magnesium carbonate. Tare. Add 0.05 grams of calcium carbonate. Note that this is only for Portland tap water as an input. Use information from your city or county's water report and this spreadsheet. Dump minerals into keg of water and reinstall lid.
Step 6: Carbonate
Add co2 gas to mineral water. Carbonation will be aided by chilling water and/or agitating. It will take 1-2 days to be fully carbonated.
Step 7: Enjoy!
Drink your newly created mineral water!