To smooth out any printing artifacts in the joint sockets for better movement. Step-by-Step Assembly Guide
If you are missing specific diagrams, you can find community advice and official creator updates on these platforms:
Thread a separate piece of elastic through one hand (secured by an S-hook), through the arm segments, across the chest/shoulder cavity, and down the other arm to the second hand.
A thin wire loop or a "hemostat" to pull the elastic through the narrow limb channels.