Like Randy said do not cut all the way through the blank, use inlays that are the same width as your blade's kerf, use a stop block on your fence or sled so you can return the blank the correct place, and finally make sure your blank is geometrically correct. For example, if a 4 sided knot is used, the blanks needs to have sides that are 90 degrees to the adjacent. Or for a round blank, marks showing you the correct orientation.