Once the balloon is filled tie a piece of string around the neck (above the still attached filler) tighten and then tie the other end of this string to the top of the helium tank. This will act as a safety line in case of a sudden gust of wind etc.
Above the string use one cable tie (a small one would probably be best) and tighten as much as possible. Do the same with a cable tie below the safety line. Try to keep them relatively close together so that there is still some excess balloon neck below.
Now while one person holds the tied neck (preferably with gloves) remove the balloon filler. If possible tighten the cable ties. Now attach the payload cord directly below the lowest cable tie with a very strong knot. Next loop the excess neck round and using another cable tie attach it to top end of the neck. Add a second cable tie for a little more strength.
Now the payload chord should be attached at the bottom of the loop of neck and the balloon ready to launch. Don't forget to shorten the excess cable ties and remove the safety line (just cut the string near the balloon and leave the part tied around the neck of the balloon.
The following is an example of how most UK HABers fill and tie off the balloon. Normally there are lots of hands in the way - so I have produced these shots using a dummy neck. This is a shot of a Hwoyee 500 balloon neck - with UK 2p piece for reference :-
1) Put the neck on the end of the fill tube - fold over any excess diameter and tie on with a cable tie. A 40mm OD fill tube is shown - this is a snug fit on Totex balloon - but much smaller than the Hwoyee necks.
2) Its also a good idea to duct tape the neck on as well - it adds a further seal and some more security against accidental release. put a tab on the end of the tape to make it easy to get off.
3) OK - now fill the balloon. Weights are normally tied to the fill tube - such that when the balloon starts to lift the weights off the ground the right amount of fill has been achieved. When its full enough tie off the neck with a couple of cable ties and clip off the ends.
4) Now tie on the line - several tight turns round the neck between the ties. I normally make several turns, tie a knot, make several more turns and tie again. Tie the other end of the line to something substantial - so that if the balloon is accidentally released it should be held on the line.
5) Holding the balloon securely, remove the tape and tie. This and the next step are the trickiest.
6) Holding the balloon by the neck above the ties fold over the excess neck and tie off with a couple of more cable ties. Try to get these ties to go over above the existing two ties. Clip these ties off close to the ratchet. This step is best done as a two man job - someone holding the balloon, someone adding the ties.
7) Finally tape over with some duct tape - this adds more security - and stops and pointy bits on the ties poking out to make a smooth place to hold the balloon by.