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I found this eventually:
https://justasmallblog.wordpress.com/2012/07/19/how-to-resize-a-seiko-collar-and-pin-watch-band/
I had thought the collar went in one of the outsides, and the pin went through the other. I could not get the collar to go in, so kept googling.
So, I put the collar in the middle part of the links, then holding the watch strap together force the pin back in. Helped to have a bit of metal to push the pin back in as it takes some force.
hah yeah once I worked out how the collar pins go, it was very easy. At least these ones fit on the middle of the link, rather than the outside. There was nothing at all in the book on how to do it!
@spunk_nautical I am really happy with it, looks better in person!