Any stick has 2 faces, 2 edges, and 2 ends or 6 sides!
If you are making a rectangle, like a picture frame with "mitered" corners it will look like this:
So, is that what you are doing? If so, just make your angles at 45 degrees. If the project is a rectangle, 2 sides will be longer than the others, unlike a square where all 4 sides are equal length.
If you are making an "8 sided stick" that's not really clear... then are you just sawing a little off the 4 corners at 45 degrees., leaving 2 faces, 2 sides, 2 ends and 4 miters, or a 10 sided stick.
If you are sawing off the corners such that there are no flat ends, just points at 45 degrees, then I understand. You have to find the center of the board, then saw the corners off to meet the centerline.