By my recommendation, my new workmate He (贺) moved to Ubuntu from windows, and he was annoyed of one thing: cannot input Chinese. And I have to google again to find out the answer. This is why I wrote this list for him like people and myself.
sudo apt install fcitx fcitx-rime im-config # and select fcitx as the input engine sudo reboot # important! fcitx-configtool # choose the correct input method you need