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 later.

Thread two M3x10 screws inside the top and bottom holes of x_end_idler. Do not screw them fully, leave a gap of 1-2mm as shown on the third image.