How do i shorten a Casio waveceptor metal watch strap please?

You may or may not have arrows inside bracelet. Arrows are pointing the way the pins must come out.
First you have to find out what sort of pins or even screws are used to keep links together.
Examine both bracelet sides and find the side where pin ends have a groove.
Start with a tiny screwdriver and unscrew one of the screws. If it turns, but doesn't come out, that means you have the pins instead of screws.
For removing pins the best would be pin removing tool (approx.5GBP on ebay), but it's possible to do the job without it.
Take a hardened steel needle and blunt the sharp end to the approx. size of pin end diameter using any sharpening stone or sandpaper. Get an old towel and fold it to make a soft base for work (like small cusion). As you don't have a special bracelet holder you will need a pair of helping hands to hold the bracelet steady upright. You will need small hammer and pair of flat nose pliers as well.
NOTE: The pins MUST be driven split end out first, not vice versa. Make sure that the grooved pin ends are facing towel, not the needle and hammer!!!
Now place the watch on the folded towel, take that needle and smallest hammer you have and start driving the pin out using light blows. Do Not hit hard, as you will brake the needle, scratch your watch or even injure yourself. Watchmakers are using 45 gram hammer, so, calculate your strength of blows.
After a few blows check if the split end is coming out and when you see that the end is long enough to grab it with pliers, pull the pin out with pliers. When pulling, do NOT turn pliers, as pins tend to brake when twisted. Use firm grip and your strength to pull the pin without twisting.
After shortening the bracelet or adding extra link(s) make sure that the pins go back EXACTLY the same way as they came out- the blunt end first in the hole and the split (grooved) end last. Before doing that check remaining pins in bracelet to make sure that you put them back from the right side of bracelet.
It is recommended to use plastic hammer to drive pins back in or you will scratch or damage bracelet. If plastic hammer is not available use an old toothbrush handle (or some plastic item) as an absorber. Simply put pin into the hole as deep as you can with your fingers, put the toothbrush handle on split pin end and hit handle, not the pin.
Make sure that pin ends are flush with bracelet. If needed- hit few more times.
Job done!
If you do not want to do it yourself, any jeweler will do it in a matter of few minutes.
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