Determine why the lake is extremely acidic. Correcting the pH might be wasted effort if nothing is done about the cause(s). If the lake, for instance, is a victim of acid rain or acid deposition from a near or distant coal-burning electric power generation plant or from the run-off from a strip mine, community action to fix the problem would be needed.
Calibrate the portable pH meter according to the manufacturer's instructions. Typically, put a base with a known pH of 4 in a disposable plastic cup and take a first reading with the glass-tipped electrode. Rinse off the electrode with distilled water. Then put a base with a known pH of 7 in a disposable plastic cup and take a second reading, then rinse the electrode with distilled water. Calibrate the meter's pH scale by these two readings.
Raise the pH of the lake by adding a neutralizing base, such as calcium carbonate (CaCO2) or powdered limestone (CaCO3), by aerial application. This is a temporary measure. Long term, unpolluted water entering the lake via rain and streams will raise the pH reading.
Make a plan of action for the lake's recovery after the pH has been brought up to a desired level. Many factors in the ecology of a lake affect its pH, and too much or too little of a factor---such as photosynthesis on the one hand and respiration and decomposition on the other---can have an adverse effect on a lake's buffering ability to withstand pH variations.
Monitor the lake's pH periodically as you implement the rest of the lake recovery plan.