Funny that you bring this up. I teach high school biology, and each year when we do botany I do a whole lesson on yucca. That was my lesson this last Friday. And then, as an encore, I did the same thing yesterday at a nature center in Arlington, TX.
I haven't used any special method for removing fibers, other than just scraping the surface with a knife blade to remove the epidermal layer, and then stripping out the fibers with my thumbnail. I am pretty certain there is a better way to do this, but it has sufficed for what I am doing. I just cord it like a two ply sinew bowstring. In a pinch this works pretty well, but I can only shoot about 20 arrows through the ones I've made, before the string breaks. I bet I haven't used a yucca string in about 20 years. When I was first making these, I was actually just practicing to make better sinew strings. Yucca strings can sure make your fingers raw if you don't wear finger tabs!
When you get one done, make sure to post your results. I am always interested in others experiences. By the way, you can also make nice baskets out of the stuff! Curtis