I soak the tail in warm water for 15 minutes before I operate. As well a good single edge razor blade is a must. I find that the wrapping is easier if the tail is still supple from the soaking.
One other factor is practice. It seems alot easier to do now that I've tied a bunch of them!
Some guys have told me they sandpaper the pith off the back of the split tail which reduces the thickness of the feather. While I haven't tried it, it makes sense. Good luck.
Tight lines - tyler.
Still Living Large!