Open up the 'Global' tab (bottom right, in between 'build' and 'research' tabs)
From the pop-up menu, use the middle shield-shaped 'Your Guild' tab.
In the window: the middle selection 'Members' window
In the member list, find your name: hit the red button on the right side ; 'Leave Guild'.
Would you like an invitation to an excellent guild, whilst we're talking...?
