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Supposed to be "squeezed" tight (on each short side of that rectangle) - like an Oetiker clamp. There are special pliers for that, but can be done with cross cut wire cutters. Just go slow, and don't overtighten.
Special pliers to tighten a clamp in a non-critical application was not something I wanted to spend money on. So I just used black zip ties, cinched snug. No need to over-complicate, IMO.
Thanks everyone. I bought these from AF1 racing, I have now emailed them suggesting that they put this in their product description, people thinking of purchasing these need to know. Oetiker clamp pliers are not a tool that most motorcyclists have in their kit and none of the large outlets here carry them, so if I wanted a pair I would have to buy them online. So, I am going to do as suggested and try and close them carefully with with a pair of cutters.