As already described, it is easy to find the the North-South direction and the Local Noon at your location. Once you have done this, you may now calculate your accurate geographical longitude. The crucial point is to measure WHEN the Sun crosses YOUR local meridian, that is when reaches its highest position and is seen due South.
These data must now be compared with the official data from Greenwich (further details below).
We will demonstrate how to do this with an Example:
Suppose that with a gnomon (or other simple equipment), you find that the Sun reaches its highest point in your place at, say, 1144 CET, that is, at 1044 GMT (remember that GMT = CES - 1 hour).
On that very same day, the Sun reaches its highest point above Greenwich at, say, 1150 GMT. Local Noon at your place is therefore exactly 1 hour 6 minutes (or 1.1 hour) earlier than in Greenwich.
This time difference corresponds to a longitude difference of 1.1 hour x 15°/hour = 16.5°.
In your place, the Sun reached its maximum height before it did in Greenwich, that is, you must be East of Greenwich. Since the longitude of Greenwich is 0°, this shows that you are at Eastern Longitude = 16.5° East.
Thus the trick is simply to know accurately, when the Sun - on the same day - reaches its maximum elevation above the horizon at your place, and also in Greenwich. You need these two times to determine your geographical longitude.
The first value must be measured by yourself. The second value may be found with a certain accuracy for instance on the web site of the Nautical Almanac. Here you have to enter the coordinates of Greenwich (that is, merely longitude = 0°), and subsequently ask for (rise, set and) transit times (this means when the Sun passes South).
Another method is by clicking here, whereby you will obtain a full 1-year almanac, as given in the author's high-school book on Sundial Math and Navigation (see the front page for this section on Astronomical Navigation).
On March 21, 1998, a school class somewhere in Europe finds that Local Noon in their town occurs at 12 hours 13 minutes CET. What is the longitude of their town? Find some towns on a map of Europe which are located near this longitude (and which may therefore be where that school is located).