Trebor,
You got a lot of information, but I think not the bit you was asking for.
That sling in the picture was made by Klahr, he wrote somewhere how he made the pocket, I re-tell this in my own words and with the information I read in the picture. He did unwind a little of the string, where he wanted to have the pocket, tied a knot at the start of the pocket and in that same knot also tied an extra strip of plastic. By unwinding the string he got four thinner strings next to each other. (If you have store bought string you will have 6.) Now he wound the plastic strip round the 4 strings, going over one, under the next, over and under till there are no more strings to pass, then go the other direction. Keep the strings in the same order, and now go under where before you went over and over where you did go under before. Go on with weaving round the strings till the pocket is long enough. You can do this weaving (sometimes called wrapping) with string too. At the end of the pocket you make an other knot, in this you tie the end of your weaving string or strip.
You can also start the sling like the split pocket sling,
http://www.slinging.org/20.html or like the sling with the leather pocket
http://www.slinging.org/13.html make the pouch like Klahr did and go on with the instructions in the article for the second half of the string.
Weaving might be easier if you tie the string between 2 fixed points, (table cairs will be enough) or one end to your body, (belt?) and the other end to a fixed point, (table?)
Succes, Willeke.
PS, Klahr, if you read this, can you please explain how you really made this pouch?