Grip the X motor shaft with pliers to prevent it moving (grab the flat side of the shaft).

Tension the belt, by tightening the screws in the x_end_idler. Do this while trying to move the extruder to the left or right. The belt should stay straight and should not bow up and skip over the drive pulley teeth.

No need to be very precise with the belt tension here as the final tension will be improved in a further step.