Yea, definitely not an easy thing to do.
I would be inclined to think that just going for it may be the best thing.
The trying to taper down thing just prolongs it and may not be any easier since your into a fast.
It may even be easier to go cold turkey while being in a fast because you can usually feel the good things that are happening with the body once you get along into the fast.
So what you feel good happening may make it easier to stay smoke free.
When I quit smoking, which I finally ended up having to do cold turkey because a year of trying to taper off just didn't cut it, I purposefully put my attention on how much better I was feeling each day without the smoke, and that was sort of the beacon that kept pulling me along.
Each day got better and I always wanted to know how much better tomorrow would be.