For a sundial to be accurate it needs to be designed differently depending upon where its located on the planet. A sundial made for Ecuador won't display the correct time in New York & visa versa. It has to do with how the sun strikes the Earth at different angles at different locations.

As you can see from this diagram, someone standing on the North Pole would see the sun low on the horizon. At the same time, someone on the Equator sees the same sun directly overhead. A New Yorker on the other hand sees it somewhere between. So when it comes to sundials, one size does not fit all.

So before you begin you need to know where on the planet you are, or more precisely, what your latitude & longitude is. There's many ways to figure this out but one of the simplest can be found here at Satellite Signal's latitude & longitude finder.