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We need a precise volume of the beer. In this case, we have 15 mL. We also need NaOH in liquid form. Typically, it is sold in 0.1M form. Now, the trickiest part of this is adding precise amounts of NaOH (say, 0.1-0.5 mL at time), to your 15 mL of beer. Every time you add NaOH, you must vigorously stir the sample so it is well-mixed. Then you can measure its pH. You continue this until you reach the desired pH baseline of 8.2.
:''Note: The baseline value of 8.2 pH is somewhat arbitrary, but it is the US and Australian industry standard. A pH of 7 is a neutral pH and the pH of water, whereas ~8.2 is near the equivalence point for a lactic acid/sodium hydroxide reaction. A pH of 8.2 is also where a titration dye, phenolphthalein, changes color. A well-calibrated pH meter is easier to use than dye, not to mention its superior accuracy and precision, if used correctly (well-calibrated, probe well-maintained, etc.). A pH of 7 is the European industry standard for measuring TA in wine <ref>[http://www.awri.com.au/wp-content/uploads//TN14.pdf "TN14 - Interconversion of acidity units" Industry Development and Support. Australian Wine Research Institute. Retrieved 09/15/2016.]</ref>.''
At or around a pH of 8.2, we have reached our equivalence point for a titration of pure NaOH and pure lactic acid. We need to convert the moles of NaOH we added into moles of lactic acid, and then divide the equivalent grams of lactic acid by the original volume of beer. That gets us g/L, and our titratable acidity. For a numerical example, assume 15mL beer, 5mL 0.1M NaOH: