Basically when the event happens (once every ~200 days though it's entirely predetermined on your gameseed), the game will select 100 tiles at random on the farm map, they can be water, cliff, farmland, on a building, or on the black edges of the map.
If any of these are free, it can place the statue there. To be free though, it has to be FREE, no paths, nothing (I think grass is fine).
If you place stuff like buildings or paths on all the applicable tiles you just won't get them anymore
You don't have to keep the farm super free if you're worried about missing it, 100 tiles is a lot and you have something like a 99% chance of getting it with only 60 tiles free (which is what my crazy coop farm has for reference)