You need sufficient acid (buffers) sources to keep the pH low, things like bog wood, peat moss, Indian almond leaves etc. Water is quite a good solvent, so if you put these things in areas with flow, they will dissolve faster and release acid faster.
pH is the equilibrium point between carbonic acid, bicarbonate and carbonate. So to shift the pH lower (equilibrium to the left) you need more acid, and to shift the pH higher (equilibrium to the right) you need more carbonate (alkaline sources).
The natural acid sources listed above will want to buffer at a low pH, so don't remove all of the carbonate sources thinking that this will help to lower pH. It will, but the equilibrium point will shift far to the left. Instead, you need to find the balance point where you have just enough carbonate sources to feed the acid sources, where the equilibrium point is at the desired pH.
An excellent source of carbonates are things like shell grit. The poor solubility of shell grit (all carbonate sources) is an advantage, since it won't all dissolve quickly, and thus provides a good stable source of carbonate. As with the acid sources, placing carbonate sources in areas of higher flow will increase the rate at which they dissolve. The higher the amount of acid in the water, the faster the shell grit will dissolve also. This helps to protect against pH crash, since as the pH lowers, the shell grit dissolves faster releasing more carbonate to offset the effects of the acid (a good thing).
A healthy supply of both acid and alkaline sources will provide stable levels of each. If you need to remove all carbonate except for one little piece of shell grit to reach the desired pH, you're probably to close to disaster, since when that one piece of shell grit dissolves, and there are no longer any carbonate sources present, pH is likely to drop very low, and very rapidly, leading to disaster. With this in mind, take things slowly and don't get overzealous about hitting a specific pH. pH is just an indicator, and the fish won't care if it's not at the optimal point, but they will care if the balance is lost and things start swinging rapidly.
Since your KH test kit is likely to give results for total alkalinity, it's useless for determining just the carbonate portion of total alkalinity. ie: It won't accurately tell you carbonate levels. With this in mind, you're going to need to use the grey matter between your ears to ensure you have a sufficient supply of carbonate.
So in summary, the safest place to start is not to remove carbonate sources, but to increase acid sources. Once you can no longer increase acid sources (for whatever reason), then start slowly reducing carbonate sources. Don't expect immediate results, add or subtract substances, then wait.
There's a good reason why people suggest to forget about targeting a specific pH.