Find the AC-PC line
The AC-PC Line
Where are the Anterior and Posterior commissure?
Below is a little schematic (from Chris Rorden's  MRIcro tutorial) of a sagittal brain view. The anterior commissure is shown in red, and the posterior commissure in yellow.
Officially, according to the Talairach Atlas rules, the AC-PC line goes from the superior surface of the anterior commissure to the center of the posterior commissure, as shown in the sketch.
The image below shows the anterior commissure on an iconic brain from the MNI series. The AC is at the virtual intersection of the red lines.
J. Talairach and P. Tournoux, "Co-planar Stereotaxic Atlas of the Human Brain: 3-Dimensional Proportional System - an Approach to Cerebral Imaging", Thieme Medical Publishers, New York, NY, 1988