Hello, Trepsik;
    Welcome.  Some of our members like to use bees wax.  I've never tried it myself but from everything I've read that would probably be the best way to protect a natural fiber sling.  Either jute or flax is a good choice.  Jute is very inexpensive, looks good, is very easy to work with, is durable, and reasonably strong.  Most of the slings I've made have been of jute.  I've never used flax but I've seen flax slings that other members have made and they're beautiful.  Paleoarts likes to use flax a lot.  If I could find some around here I'd try it.   I've also used hemp and like it for many of the same reasons that I like jute.  Sisal will make you a good sling but it can be very scratchy and rough on your hands and difficult to work with.  Cotton is a very good choice too.  It's light, strong, looks great, and is easy to work with.  Some members like silk and wool but I've yet to try them.

Welcome to the site Trepsik. Look forward to seeing the finished product. Hemp makes a good sling. Repeated soakings in hot water and then wringing out and letting dry will make the cords very supple. Never needed beeswax for hemp as it tends to be much smoother than jute but it can't hurt either. Good luck.

Welcome Trepsik!  I've not much to add, I have used all of the above mentioned natural fibers and have only had trouble with sisal.  I have never used beeswax, and I don't think it  is necessary, but some do use it and I suppose it would help in the long (long long) run.  Burning off loose fibers is a matter of opinion, they do not hurt anything but it you don't like them, burn them off.
Soaking a completed sling will help to brake it in, it makes it very supple and nice, but I would not soak natural fibers before you braid them because they tend to unwind.
Good luck with your sling.

everything above, and add whipping ot the base of the pouch on the release cord. when the stone leaves the pouch it tends to scrape the first bit of the release cord. whipping protects the cord from being worn through. depending on your style, you may also have to whip the end of the release cord as well, as this part generally hits the ground in overhand styles, and receives the most stress.

My only sling-care and -maintainance: hang up and let it dry out if it`s wet, rinse in clean water first if it`s muddy too

Compared to almost everything else I've used sisal has the most elastic memory.

by elastic memory do you mean the amount of twisting in the end product? thats what i meant, i usually untwist the strand, and begin braiding, and it turns out much less torqued than jute or cotton string, i need to learn some more slingning terminology...