How To Make A Sundial

If you set a vertical sundial in the Northern hemisphere, it has to face South. A compass tells you magnetic North. If you spin 180°, you face magnetic South. So, you have to determine geographic South.

It is easy to do, thought it takes a little time.
Get up before noon and go out in the garden. Set up a long and fine rod upright, perpendicularly to the ground. Its shadow is cast on a level ground. Mark the tip of the shadow. Measure the length between the stick and your mark. Repeat every quarter of an hour. At midday, the shadow must be the shortest, and then it starts stretching again.
In the afternoon, when the length of the shadow is the same length as the first one your measured, draw a line between the two marks. Draw a perpendicular line (= a line that makes 2 right angles) towards the stick: it points to true South.
Set the sundial. The shadow of the edge of the gnomon tells time.

The vertical sundial has to stand facing South. Most people nail it against a wall of the house. Since it is rare that a house has the exact orientation you need, it is often possible to add a wedge between the dial and the wall so that the sundial faces the right direction.

A very decorative way of setting a vertical sundial: the arch. You make stand an arch (any strong frame will do the trick) along an imaginary line that goes West-East, so that the dial hanging from its top will be oriented North-South.
When hanging that way, a vertical sundial shows two faces. People usually think that the face that looks towards South is the only one that tells time; but the sun of the earliest and of the latest hours of the Summer days casts a shadow on the Northern dial as well, providing that a style has been fixed on it.

Of course, the style (gnomon) is as important as the dial itself. Take care, when you adorn it, not to change its inclination.