The thing I don't understand is how does finding out the median help you drawing in the horizontal and vertical lines. How do you know if it's going to pass through a few of the data points? Like in the example on that website it has a moderate negative correlation and the median is the 8'th score. How did they put the vertical+horizontal line by knowing the median? I see 6 data points in the B quadrant, 5 in the D quadrant, 1 in both A and C. How did they know all this by just the 8'th score? Thanks.

Look at the points in order from left to right. The x-coordinates are in increasing order, so locating the median can be done by counting.

If you look at the points from bottom to top, the y-coordinates are in increasing order, so you know where the median is.

Drawing the two lines will generate the four quadrants. (Note: the vertical line will always pass through any point with an x-coordinate equal to the median, and the horizontal line will always pass through any point with a y-coordinate equal to that median. These may not be the same point.)