Casting this spell draws on the knowledge of all creatures within the range, and poses a subconscious question to these creatures: Where is the landmark in question. The answer is then returned to the caster as long as the location is on the current plane of existence, and there is a general consensus from the creatures in the area. Landmarks must be stationary and singular. Should a stationary landmark be moved, perhaps the "Great Tree of Vitality" has been cut down and moved, the original location of the landmark as it is known to people in the range of the spell is used. Mobile landmarks, like a King's Crown, automatically fail.
In preparation for this spell, an 10' diameter area must be prepared. If an area has already been prepared (such as a dedicated divination room within a wizard's lair), the spell can be cast straight away taking only 4 hours. If such an area must be constructed, the area must be made uniform and neat. A wheel must be carefully inscribed on the outside with radial spikes to the center. This can be etched, painted, or even made with sticks in a pinch, but the preparation and placement must be exact. In optimal conditions, this takes 2 hours (e.g. paint on an empty warehouse floor). In ordinary conditions, this takes 4 hours (e.g. etching on flat stone). In poor conditions, this takes 6 hours (e.g. sticks and leaves in the forest floor). If the caster is in a hurry, they may use half the prep time with a successful intelligence check. If the check is failed, the caster has not prepared the area correctly and the spell flubs. Generally it is best to roll the intelligence as the spell is being cast.
The actual casting of the spell takes 4 hours, during which time the caster must sit still in the center of the wheel with their eyes closed and their mind in an open state. They cannot carry conversation or interact with objects without flubbing the spell. The magic sweeps the area in an apparently haphazard manner before returning to the caster a direction (if the spell is successful).
Example: Bobo the diviner casts Locate Landmark, with the landmark in question being the secret burial ground of elves. Bobo is in a human dominated region, where nobody knows the answer to this question, so it returns no answer. Bobo then decides to head to a major port where there are many elves, and casts the spell again. The compass in his hand turns to point North West. Bobo doesn't know how far North West it is, so he travels North West for six days before casting the spell again. This time he is in a small elvan port town that allows human visitors. He casts the spell again, and gets a direction of south west. Bobo knows he's overshot the distance, and can change directions, heading south this time.
A very careful wizard with a compass and map might be able to triangulate the location of a landmark, although this can be tricky since the further the target is from the landmark, the more the compass needle wobbles.