Move the X axis in the middle and shutdown the printer.

Tighten the belt until the belt start to be straight. Don't tighten too much, you should still be able to pinch the belt with very low force.

Unscrew the pulley set screws a little.

Adjust position of the pulley to have the belt centered. You can move the extruder to verify.

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

Tighten alternatively the set screws again.

Block the X motor shaft with pliers (grab the flat side of the shaft). Tension the belt while moving the extruder left and right until it is resisting. The belt should stay straight and not deform.