Hi I'm a beginner slinger and sling maker from Barcelona. I made a couple of simple quick 2 strand slings half a year ago but recently i bought some paracord and started making some serious ones and I'm pretty hooked. I'm impressed with the potential of this primitive weapon and looking forward to gain consistency and technique. I watched a lot of balearic slingers because i really like that cast tecnique and want to learn it, im under the impression that its one of the more powerful. Atm I'm using a kind of intuitive helicopter cast but stil learning. Here is a picture of my slings so far. The source material I used is:

-The Weavers of Eternity Paracord Tutorials for the white one, although the pouch is very rigid and flat and not very efective.
-CodeMaker: Single strand Paracord split pouch sling for the black one , with an adjustable string piece in the middle for adjusting the aperture similar to what Practical paracord does.
-And the leather one taking Môn Slinger's pattern which ended up a little small but i like the pouch a lot.

I need to learn more about braiding because i want to make a primitve rock sling with natural materials, and one questions i have is what thickness should I aim for in the sling cord. Also i found that if I only use 1 finger for the retention loop it ends up hurting me so I'll try puting 2 fingers (middle and ring) in the loop.

Welcome from Pennsylvania, USA!

It's difficult to answer the question of how thick sling cords should be.  A lot of factors come into play.  The style you're using, what kind of projectiles you're throwing, and also just what you prefer.  I think the advice you'll get from a lot of folks here is to try out some different levels of thickness and see what you like.  Which would be my recommendation as well.

And your description of using a loop that goes around your ring and middle finger is exactly what I do.  The first sling I ever made had a loop that was too big, so I put two fingers through it.  Now I have trouble using any other method.

Welcome Vrod.
Good sling 
For me, I don't use a finger loop and it's ok.

Welcome from northern maine

Welcome from New Jersey, paintman;
    Nice slings.  One of my favorite materials for weaving is jute.  It has very little memory so it's easy to work with, it makes a great looking sling, it holds up reasonably  well for a natural fiber, and it's as cheap as dirt.  Hemp  is good too for the same reasons.  I've made some nice slings from cotton also. 
    I've always used my ring finger in the retention loop.  Everyone is different.  What ever works for you is the proper way.