Click on the link below to follow a short clip.
How to tie a double overhand loop knot.
Another knot that may be of use is the 'sliding slip knot'. This allows your string/cord length to shortened once you put you necklace on. It is also known as the 'double fisherman's knot'
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This link measures in inches as the demonstration is from America. It is easy to convert these measurements. Check out the Multicraft inch/mm rulers.