Move the extruder to the middle of the X axis.

Pluck the lower belt to make it vibrate. Don't pluck it 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 adjustment, move the extruder to all the way to the left and right and then center it again.

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

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