Clamp the extruder on the top and bottom linear bearings of the X axis

Make sure to not let fall down the extruder until it is secured, keep an hand on it.

Make sure all cables, except the hotend cables, are running on top of the bottom smooth rod (between two X smooth rods). The hotend cables should stay under the bottom smooth rod.

Clamp the top of the x_carriage_back.

Secure it with two M3x10 screws in the top holes. Don't tighten them yet!

Finish securing it with two M3x18 screws in the bottom holes. Don't tighten them yet!

Alternately tighten the four screws while moving the extruder. Don't over-tighten them.