Just hold the synthetic material bundle for your tail in one hand and then with the fingers of your other hand grasp some fibers from the center of the bundle and pull to get those fibers longer than the others of the bundle. You may have to do this operation more than once to get the taper you desire. Basically you are trying to create a tailing bundle similar to what you get when you cut of a bundle of bucktail from the tail. Trimming with scissors may be required to clean up some uncooperative fibers.