Move the X axis to the middle of the Z axis and shutdown the printer.

Tighten the belt until the belt is straight and doesn't sag. Don't tighten too much, you should still be able to pinch the belt with very little force force.

Loosen the two pulley set screws a little.

Adjust position of the pulley to have the belt centred. You can move the extruder by hand, to the left and right, to verify that the belt is centred.

Tighten the set screws alternately.

Verify that the belt is not touching the x_end_motor and that the drive pulley is not touching the X motor.