The first thing that you have to do to make this pen is decide what pen kit you are going to use so you can size the disign properly and put it to scale.  What different woods you are going to use and their color contrasts to each other to make this pen stand out.  Accents such as Celtic rings have to be figured out for the size of the blank and also matching color,  what metals to use and what glues to bond these metals to wood as different woods do not bond to metal as good as others.

What size are your pieces going to be and how many are you going to have to use.  How many layers will it take and still get all your colors to show on the pen. 

All in all there is well over a hundred ways you can fail at making a perfect Herringbone 360 blank.  I have made around twenty of these since I started and I am still finding ways to improve the process and avoid disaster.