Install the z_tops on top of the Z axis. T-nuts must go into the extrusion's grooves and the smooth rods in the smallest of the two front holes.

You must press down the z_tops until the smooth rod is touching the top of the hole.

This is very important as it might affect the alignment of your X axis later.

Make sure the z_tops are parallel to the top of the extrusion

Tighten the 2x M5x10 screws.

Double check that the smooth rods are fully inserted in the z_tops.