Unlatch the buckle. Press the release on the side of your watch's buckle so that it opens. Locate the release pin at the edge of the buckle to remove it from the rest of the clasp. Poke the tip of the watchband sizing tool into the release pin and pop it out. The watchband is now opened to lay flat. Put the pin to the side for later and lay the band flat.
Remove the buckle. Use the screwdriver to unscrew the buckle from the band. If the screws won't give or continue to spin in place, put another screwdriver or the sizing tool in the other side of the screw to create more force when you use the screwdriver. Some watches have a release pin instead of a screw. If that's the case, use the sizing tool the same way you did to unlatch the buckle. Put the buckle and any pins or screws to the side.
Remove any links. Starting at the edge of the band where the buckle used to be, determine how many links you need to take out by wrapping the watch back around your wrist with the removed buckle in place. Unscrew the links with the screwdriver. If the screws stick or spin, use the same technique of placing another screwdriver or the sizing tool into the opposite side as in step 2.
Put the buckle back on. If the buckle attaches with a pushpin, reinsert the pushpin. If the pin is too delicate for you to get back in with your fingers, use a pair of tweezers to handle it and push it back in with the sizing tool. If it screws in, use the screwdriver. Reconnect the buckle to the rest of the clasp by reinserting the pushpin the same way.