[Qlong]

Some parts are press fit, requires a mallet or a press. I understand that you are trying to learn, no problem with that, but it seems like you are lacking some fundamental technical experience.

The bearings should seat onto post, then the screw inserted into it, where the head of the screw will sit flush against the countersunk post. In other words, turn the "last piece" the other way.

Grip the gold piston head and start driving in the screw. The piston head piston will self-press.