Calibration buffers are solutions of salt dissolved in pure water with known pH values at varying temperatures. Since the apparent pH value of each buffer is temperature-dependent, it's important to calibrate the logger at a temperature that's similar to that of the sample.
When your sample is greater than 25°C, the best way to match the temperature of the sample is to use a water bath. Place the calibration solutions in individual beakers and place them into a water bath with the temperature set to that of the sample. Make sure not to place buffers on direct heat or to allow the water in them to evaporate, as this will change the quality and condition of the buffer.
When your sample is less than 25°C, you can match the temperature of the sample by storing the buffers in a refrigerator that's close to the temperature of the sample, or you can use a water bath as described above.