Grab the flathead that fits and unscrew the desire links. After that you screw back the links and try to fit your wrist. If don't work you repeat. No need for in dept tutorial.
If your going to be into watches for the long haul, a good set of watchmaker screwdrivers are worth the investment. You will use them at minimum everytime you get a new watch to resize. One of the dealers here, Offshore... sells a great product for small money.
Have a look here, this is his site
http://watchbitz.com.au/shop/index.php?_a=viewCat&catId=15
Well done!