There is no simple rule of thumb for how much acid you need to add to water, it can vary by a factor of a hundred or more depending on the hardness of your water. You can buy test kits which will tell you the total hardness or alkalinity of your water and you could calculate the amount of acid needed to reach a particular pH, but it is simpler just to titrate it. Add some Universal Indicator, then add your acid until the pH reaches about 5. That's how much you will need in future. pH 5 is about what rainwater would be in the absence of other contaminants. Acidifying much less than that would be a waste of time, acidifying much more could damage plants. Watch out for water supplies which vary during the year as water is drawn from different sources. Remember that acidified hard water is not the same as rainwater, distilled water, de-ionised water, or other types of pure water. It will still contain high levels of minerals, just no longer alkaline minerals. You may still get issues with clogging or salt buildup and non-carbonate minerals like iron may still cause problems like staining.
Professionals tend to acidify their water using sulphuric acid. It is relatively cheap, strong, and the by-products are not toxic to plants. It may be difficult to obtain and can also be dangerous, so you might want to use vinegar instead. Phosphoric acid is also an option, but be aware of the extra phosphate that you will be applying.
For semi-permanent modification of soil pH in open ground, elemental sulphur is the only real option. It works very slowly over a period of months to years as it is slowly converted to acid, but is more effective than an equal weight of any other chemical you can apply. It is slowly oxidised by soil bacteria, then hopefully dissolves in soil moisture to produce a reactive acid. Obviously it needs to be applied below the surface in moist soil to be most effective. Again, professionals have used direct injection of sulphuric acid to acidify soil immediately, but without further treatment the soil will almost certainly revert to near its original pH. Sulphur treatment is not suitable for irrigation or use in pots except in very small quantities. In pots you should be using a soil already at a suitable pH.
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