Gather the 200g weight

M3 nut

M3x15 screw

Weight holder

Weight retainer

and your previously assembled parts

Fit the weight into the holder and slide the retrainer over top.

Attach the retainer to the rest of the pen holder with the M3 nylock nut and the M3x15 screw. The retainer should still be able to move, so don't over-tighten.