Many of the tips I have read regarding starting to round a spindle on a lathe recommend starting with octagonal stock. Many of the tool handles on tools I like, have octagonal handles. So I thought I would define the simplest method I have found for making octagonal stock from square stock.
Be sure you have square to begin with.
Find the center of the square.
Take the length from corner to center and transfer it to the side face.
Set your marking gauge to that length and drag it down each face.
Flip the stock end for end and repeat the marking gauge down each face.
You should now have two guidelines on each face. These guidelines are where you will plane down to as you remove the corners of the square.
Secure the stock on its corner either in a vice or using a long V-block
Use your ripping plane (a course set plane of almost any type will work. (I use my scrub plane)
Stop ripping when you get down to the line (you might want to switch to a finer set plane when you get close)
For more details on this method and 4 other less fast methods of laying out an octagon on larger stock can be found on making stock octagonal.
-- Galootish log blog, http://www.timberframe-tools.com