i am aware that there is already a string sling tutorial here:
http://slinging.org/42.html
but i was unable to figure out the knots for this sling until recently, and recognizing the usefull properties of being able to tie a sling out of a peice of string, i ventured out to try to figure out a knot that would work as a substitute.  the results are as follows.

i prefer this to the one that was previously established, because i think that it is simpler and easier to tie, once you get the hang of it. 

you start out with loops like this, as long as you want the pocket to be.



i laid the strings on the ground for this part, because it is easier to see what is going on this way, but i actaully do all of this in my hand.
you then pull the cord around the top loop and make another loop above it. 



you take the end over the rest of the string, and...



take it through the loop...



and pull it tight.  you may have to slide the knot to the end of the loop, buit either way  it is easy to do this step.



you turn the thing around and repeat on the other side.







you can tie a simple knot like this for the finger loop;



or you can tie this kind of knot(the name escapes me at the moment);










for the release knot, i use a figure eight, as it is bigger than a usual overhand knot, and it can be undone pretty easily;







start with an overhand knot, then add an extra twist to the cross, and pull the end of the tring through.


here is a giant sling i made with a 1 cm diameter rope.  turns out it makes a pretty good swing. 



i am in this one to show the size comparison.  i am 5' 5" or 165 cm tall.



here is my brother sitting in it.  sorry he is sortof low; i couln't hold him up for as long as i was waiting for the camera's timer went off.

yes, exactly.  i find that the knots actaully do come out pretty easily, depending on the string you use.   and the other one will come out as a slipknot unless you cross the cords during the tying proccess, then if you try to slip it out, it ends up as a little knot in your cord, and you have to pry it out.  thanks

yeah, that is too bad.  oh well, for the future guys who come around looking to make a sling out of a single cord.

i have made one big enough to sling a basketball, but i never had the chance to try it.  you can bet that sling  had a wrist loop instead of a finger loop. 

i just returned from a nice long walk out in the woods, and i took my big rope along with me.  so we came to a clearing that had a big pine tree in it, and i made a that giant sling again, but without the loop and retention node.  i tied it up on a horizontal branch, and then sat back and relaxed. it sortof hurt my rear end, but it was okay.  maybe if i wove another rope into the pouch as with willeke's mini, it would be more comfortable.

i've personally never tried to make a sling large enough to throw basketballs, footballs or brothers (i'd LOVE one of the brother firing slings, just give me the weight to range ratio and the right cord: and he'll be gone for a while ) thanks for the pix
Tito

btw: no offence to your brother, but he looks a bit more female than male