Mix pH Buffer powder 4.01 with 250 ml (2½ dl) of clean water. After a while, the pH of the water is 4.01, at 25 degrees. You will get the most accurate result when using distilled water, but the powder works accurately (+/- 0.1) even with normal pure water.
Calibrate the pH meter regularly with pH buffer. I first calibrated the meter to a pH value of 6.86. You can then check the calibration of the meter with a calibration fluid made of pH buffer 4.01 powder. Adjust the pH meter to mach the pH of the calibration solution value. Note that the temperature of the water affects its pH and the effect of temperature on the pH is recorded in the powder bag.