Extract the ZIP file to your Lua folder.
Switcher uses Lua actors for communication. Switcher must be running on all characters to support display of information and switching. You can use the broadcast mechanism of your choice to start the program on your characters, e.g.
/bcaa //lua run switcher
Or, to have the program loaded automatically on all characters add this command to ingame.cfg:
/lua run switcher
Hovering over the buttons shows help on what they do.