You use it the same way you would use any “pH down”. You use the new mixture to add a little at a time to your water so that it is at the correct pH before you water your plants with it or you add it to your nutrient mix to make sure the pH is in the correct range and close to your ideal target pH, 6.5 for soil and 5.8 for hydro, before feeding
Same thing for feeding. You need to be monitoring both the pH and the nutrient concentration of what you feed them as well as the pH of just the plain water. You also need to be aware of what is building up in your soil, so you don’t over feed them and to be sure the pH isn’t changing, it can have minerals build up that can make it more alkali than what you are feeding or watering it with, and also biological activity in the root zone and/or natural decomposition can sometimes make the root zone more acidic than what you are feeding it.
This is why you should be testing the soil or growing medium and/or the run-off after feeding/watering to give you an idea of what may be building up in the root zone as maybe being different than what you are feeding/watering them with.
If the nutrients or pH are way off, flushing with the correct pH water, with at least three times the volume of soil, should both flush excessive build up of of nutrient salt and bring the pH more towards the ideal. If need be, you can bring the pH of the water as low as 5.5 to make a more dramatic impact on a very high pH. And you can flush with more than 3 times the volume of soil if you need to, just be sure to let the soil nearly dry completely after all this harsh treatment to be sure the roots recover and don’t drown, before the next feeding with the recommended nutrient concentration for the stage of growth it is in.