Download and install the Pano Tuner application (developed by Kaleloft LLC) on your smartphone. Android Google Play link. Apple Store link.

Turn off the printer (if your hotend is higher than 50°C, let the hotend cool down first until the hotend fan stops spinning)

Launch the Pano Tuner app and place your smartphone on the heated bed. Make sure there is no noise in the room that could disrupt the measure (like a ceiling fan).

Move the extruder to the middle of the X axis.

Pinch the lower belt to make it vibrate. Don't pinch too much as it might touch the carriage and produce a wrong frequency

Adjust the belt tension until you have a value around 90Hz. Between each measurements, move the extruder to full left and right and then center it again.

It is very important to move left and right the extruder between each measures to relax some tension in the belt.

If your belt is new, you can repeat this procedure after few prints. A new belt will loosen on the first hour of use.