Anyone can build a very simple sundial, but it takes some knowledge of theory to build a very accurate one. I have a book on the subject: "Sundials: Their Theory and Construction" by Albert E Waugh. I have read enough of the book to sort of grasp the theory—at least to understand why building an accurate sundial isn't as simple as it might appear—but I haven't actually built one. I hope to someday.

The complication is mainly due to the fact that the earth's path around the sun is not circular but elliptical and the earth moves faster when it is closer to the sun, making a solar day longer or shorter than an average day depending on the time of year. In brief, a simple sundial that is calibrated for an average day will be off by nearly 17 minutes during part of the year. See this article at Wikipedia called Equation of time.