Use this information at your own risk. This type of sling has only been tried and used with parachute cord, use naturals at your own risk. I normally use two 5 foot sections of cord to make these it will produce one that measures about 45 inches from the center of pocket. The white string is the string the knot will be sliding along. If anyone has anything to add to the design or questions please feel free to speak up. I compared this knot to a noose earlier, it is not. There is a version however that uses a noose to make the pocket. My apologies for the confusion.
These pics should be a little easier to see using black and white string.




Use the end that you just passed through the loop to tie the last knot for that side.


Repeat the process for the other side using the sliding string to tie the knot and add your choice of loop and stopper knot. Trim and burn all loose ends and you're done.

MOST of all make certain your knots are tight and you seat your projectile firmly before you sling it.
As always your mileage may vary and questions are welcome.