I made a little post about this somewhere.  I was experimenting, and came to the idea of using the same technique the Iroquois used for weaving wampum belts.  Take a thin sapling, not too long, about as long as you want your sling.  Braid one entire cord on the sling, and then stop at the pouch.  Tie the braided cord to one end of the sapling, and tie the unbraided sling strands to the other end, making sure it is taut like a bow (not too taut, otherwise the fibres might break.)  Take one or two combs, depending on the sling pouch design (rectangular [two combs] or diamond [one comb]) and put them into the fibres to separate them out into a braid-able shape, using the teeth on the comb to separate them out well.  Keep the comb(s) in place, and start weaving in your weft, trying to keep the weave as tight as possible.  If you manage that, then maybe you could try a twining weave on your next sling (I haven't done that yet, but I intend to sometime.)  I hope these instructions were as clear as I wanted them to be, but I doubt it.  I'd suggest going to http://www.nativetech.org/wampum/wamphist.htm because there is a very good image of a wampum weaving stick.

Woven pouches are surprisingly easy.  I'd start with a minimum of 16 strands (I used 32 to make mine).  Divide them into 4 equal groups, or 'ribs'.  Take another, much longer strand, and whip it (wrap it around) the retention cord.  then just weave it over/under/over/under, coming back around when you get to the last rib.  every time you cross your weaving strand across all four ribs, smush that row back up against the previous rows.  If you do it correctly, the strands you use to make the ribs won't show through, as they'll be completely covered by the weaving strand.  Hope that makes sense.

I guess I need to make a clarification.  Once you've woven a section of the pouch about an inch long, then you split the 4 ribs into 8 ribs.  Continue to weave over/under 4 on one side, making a strap about 2-3 inches long.  Then weave another cord over/under the other four ribs 'til it matches the first strap for length.  Then you re-combine the 8 ribs back into 4 ribs, weave for another inch, and then take the strands making the ribs and continue braiding your release cord.  No need to use larger/smaller strands.

This is great information, as I have a particular interest in woven pouches.  But here's my question:

It's obvious one would need to use at least 16 (or better, 32) strands to make this work well.  What, then, is the best way to form the retention and release cords? A simple "round braid" would tend to become very wide and cumbersome, wouldn't it?

Cool.  Thanks, Wild.

I have a sling in a weaving jig I knocked up (a comb and two points to suspend the sling from) upstairs.  I have not quite done half the pouch - it is intended to end up as an "Egyptian" or middle east style sling pouch, with a diamond shape, &c.  A woven sling can actually be done quite quickly.  I only started it about half an hour ago, with breaks and coming on here to write this.

That is actually a very good question.  It was bugging me to begin with.  I came up with a solution which works ok, and produces nice pouches.  When making the "diamond" pouch, once I hit the middle, I move the strands closer together in the comb - this naturally narrows the pouch.  Then, I might loosen the hold on the strands, so that the loose fibres will come closer together when you pull the weft - this can be quite difficult to control.  The main drawback is that the "diamond" shape in the middle can fold inwards slightly, although I consider this to be a little better for slinging than a simple flat diamond.  Try to make up your own solutions.  My own works, but it is difficult and sometimes doesn't work great.
  With the Peruvian split pouches, I just move the comb along a little, as the pouch can be just about rectangular, but at the ends naturally it has to narrow.  So I loosen the strands as described above, and pull the weft almost as far as I can.  That way you get nice triangular ends.