Turn off the printer and center the extruder.

If you have applied tension to the belt, unscrew the belt tensioner on x_end_idler until the belt is relaxed.

Loosen the screws which secure the trapezoidal nuts. This is to ensure that the Trapezoidal nuts 'self-centre' on the lead screws.

Make sure the trapezoidal nuts are moving freely.

Alternate between all 4 screws evenly while tightening (alternate between both sides during the process). Don't apply any lateral force on the trapezoidal nuts.