The meeting point is next to the central fountain in the small Santiago Park (Parque de Santiago). It's a shady spot with seating should you arrive early, and the neighbouring market has a public toilet.
The end point is at the central Plaza Grande in the heart of Mérida. Your guide can help you to find transport back to your accommodation if you wish. The tour duration is approximately 3.5 to 4 hours depending on the pace of the group.