I have often reduced the size of my stainless steel watchbands in the past, using a fine-pointed punch and needle-nose pliers.  But that way has minor risks.  I don't want the hassle (or minor cost) of having a jeweler do this for me.  So I've just used this tool to quickly, easily and without risk of damaging the band to re-size a band with round pins holding the links together.

This is easy to use and easy to avoid bending the tool's pin by simply aligning the tool-pin with the small hole the watchband pin sets in.  Do this by feel before you tighten-down the tool and the watchband pin pops out easily into a slot in the tool.  You can't really see to align the pin, so you have to do it by feel.

Quality of this product is crude and it's clunky looking, but it does its job effectively and is a lot less hassle than taking the watch to a jeweler.  Also it comes with five extra pins, which I now realize I'm unlikely to need.

This tool does not seem to fit or work on Casio bands that use wide, flat "pins" to hold the links together.

The quality is good enough to accomplish its job and its very inexpensive.  Just line the pins up by feel and you'll do fine.