It's very easy to get once your population goes up to to the threashold for monuments. This can vary from map to map and is determined by the usable land you can build your city on. It can be between 70,000 to 90,000 inhabitants.
You can see your progress in the milestones window. To access this, hold down
and go to the 8 o'clock position in the radial.
Also keep a look on your city population. Once it hits and surpasses the given map threshold the achievement will unlock.
Also highlighting the (black) monument symbol in the task bar will show you a tool tip at which threshold you can expect to unlock monuments.