Insert both rods simultaneously into the x_end_idler and provide even pressure to force the rods all the way in.

Ensure that the smooth rods are fully inserted by checking the little windows on top and bottom.

Take the time to double check observation windows on both x_end_idler and x_end_motor.

If it is too hard to insert the smooth rods you can use a quality metal drill of 8mm and hand drill the first centimeter. Warning: don't drill the hole up to the end, only the first centimeter.