Learn how to calibrate your RTL-SDR dongle's PPM error value with rtl_test and kalibrate
Get a rough estimation of your PPM error
Let rtl_test run a few minutes to warm up your dongle and settle down to a rough estimation of your ppm error.
This is required for kalibrate, otherwise if your value is too far off you will not get a result.
Acquire a GSM frequency for calibration
Either look up some online database for your location or use kalibrate's scan feature:
kalibrate/src/kal -s GSM -e <rough_PPM_value>
Take the most powerful channel and use it for a calibration run (e.g. channel 86, ppm estimation 36)
kalibrate/src/kal -c <channel> -e <rough_PPM_value> kalibrate/src/kal -c 86 -e 36