10/20 Electrode Positioning System:
This system provides a standardized method of comparing electrode positioning between subjects. The locations of the markers are determined using landmarks known as the nasion, the depressed area located just above the bridge of the nose, and the inion, identified by the noticable buldge on the back of the skull.
Step 1: Using the centimeter sider of a tape measurer, measure over the center line from the Nasion to the Inion and note the length.
Step 2: Take 50% of this length and mark it as your preliminary Cz location.
Step 3: Locate your pre-auricular point by running your finger along the anterior of your ear. The pre-auricular point is the indentation directly above the zygomatic notch. Opening and closing your mouth makes it easier to locate this point.
Step 4: Using the centimeter side of your tape measurer, measure the distance from the left pre-auricular point to the right pre-auricular point and note the length.
Step 5: The intersection of these two points is the vertex of your skull and your Cz location.
From this location it is than possible to navigate your scalp according to the standardized 10-20 EEG map. The below instructional manual from TCT (Trans Cranial Technologies) provides an in depth electrode placement guide.