If your doing a 5 gal. batch in r/o water start with a 1/4 tsp in your water that your going to use for your mash. Then check your ph

Label on bottle says Phosphoric acid 10% 4oz. No instructions with mine (unless I threw them away with the packaging like a dunce). This is a standard solution as is Lactic acid at 88%. Lactic-88% is stronger and will drop your mash pH farther. Phosphoric acid is better when you need to slightly tweak pH less - maybe just nudge it a tenth or 2 of a pH. This is more flavor neutral than Lactic-88%. I own both but have never used either - its something you want to have on hand in case you miss your pH on mash day. There are lots of web resources on how to use this to hit your mash pH - and they can get quite technical really fast. You'll also want a pH meter - or pH strips you trust. Full disclosure I hit most of my mash pHs using the Bru'in water spreadsheet and Weyermann acidulated malt (another must-have item). I've only missed once and that was in the other direction - I needed less acid not more - so I used chalk not acid. I still recommend having this on hand to anyone mashing often - it's cheap insurance on brewday

Best solution is to get your water analyzed first (like Ward's water chemistry) then use something like brewersfriend water-chemistry to determine how to modify your brewing water.

Gordon Strong per his book "Modern Homebrew Recipes" adds 1/4 tsp of 10% Phosphoric Acid per 5 gal of RO brewing water to adjust the pH.

There are no real instructions since the volume needed depends on chemistry of water being used and beer recipe.

The bottle does come with a printed label that has the product information. The dosage rate will vary based on your application and water profile.