Leaving a guild in Classic WoW is actually really easy. While many people join a guild in the hopes of playing with others and cooperating towards raids and maybe PvPing, some people prefer to play alone.
If you find yourself looking to leave a guild and struggled through the social options like myself, stop right there and simply open chat.
In the chat type “/gquit” and you will be removed from whatever guild you’re in. Make sure that you actually want to leave, as some guild moderators may not be quick to invite you back if you left for no reason.