Part of the problem with making a sundial is accuracy. You have to take into account your location, elevation, and in the case of a vertical sundial, the direction its facing. There are ways to make the calculations (which I’m sure I’ll get into at some point), but now there’s no reason to. Damia.net has a web app that interfaces with Google Maps to get most of the information you need to make a sundial, but then does all the calculations for you and generates the sundial. All you have to do is print it out! Granted, this tool isn’t helpful at all after the society-destroying event finally occurs and there is no more Internet, but it could be helpful to make one in preparation!
Create your vertical sundial [Damia.net]