SparticleBrane
Contributor
I had a request for advice on tying a regulator necklace. After a few attempts at changing the camera angle, I came out with this wonderfully low-quality poorly-lit video of me tying a regulator necklace. 
Thinking this might be helpful to anyone who has questions I decided to post it here on YouTube. Click that to go to the full webpage to watch the high(er)-quality video, or just watch the normal YouTube quality video below.
Note that I'm not here to debate whether or not we should use bungie or surgical tubing, or whether or not you should use this method or zip-tie it to your mouthpiece -- I think either will work equally as well. I've never had this method come undone accidentally; it can be tightened very tightly around the reg.
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Obviously you will need a piece of bungie, the length of which will vary depending on neck size and how far you prefer your backup reg to hang below your neck.
Holding the bungie so that each end is in each of your hands with the end in your left hand longer and further from your body, pass that end under your forefinger on your left hand and wrap it around your finger and the other end twice. Now remove your finger and pass the end through the hole created from removing your finger, from left to right. Tighten as necessary, and try to avoid leaving a large 'tail' leftover after tying the knot. Repeat on the other side. Put it on your reg and crank it down nice and tight. If anyone cares, this is a double fisherman's bend.
The knot should hold tightly but you should be able to change the size of the hole for the reg without too much effort. A little bit of friction but not impossible.
You might have to try a few attempts to get one exactly the right length. Cutting an inch off will make a big difference. Make sure the ends are melted so the cord won't unravel.
If you have any questions feel free to ask!

Thinking this might be helpful to anyone who has questions I decided to post it here on YouTube. Click that to go to the full webpage to watch the high(er)-quality video, or just watch the normal YouTube quality video below.
Note that I'm not here to debate whether or not we should use bungie or surgical tubing, or whether or not you should use this method or zip-tie it to your mouthpiece -- I think either will work equally as well. I've never had this method come undone accidentally; it can be tightened very tightly around the reg.
---------------------------------------------------------------------
Obviously you will need a piece of bungie, the length of which will vary depending on neck size and how far you prefer your backup reg to hang below your neck.
Holding the bungie so that each end is in each of your hands with the end in your left hand longer and further from your body, pass that end under your forefinger on your left hand and wrap it around your finger and the other end twice. Now remove your finger and pass the end through the hole created from removing your finger, from left to right. Tighten as necessary, and try to avoid leaving a large 'tail' leftover after tying the knot. Repeat on the other side. Put it on your reg and crank it down nice and tight. If anyone cares, this is a double fisherman's bend.
The knot should hold tightly but you should be able to change the size of the hole for the reg without too much effort. A little bit of friction but not impossible.
You might have to try a few attempts to get one exactly the right length. Cutting an inch off will make a big difference. Make sure the ends are melted so the cord won't unravel.

If you have any questions feel free to ask!