This is one of my favorite tools. Make sure not to screw the punch down all the way or it will mar your metal and leave a small circle around your hole. Unscrew the punch all the way before removing the metal or you will pull on the punch and weaken the tool. Don't try to punch through any metal which is too "hard".
Make sure the little circle of metal removed by the punch has popped out of the tool and isn't clogging up the hole. If it doesn't drop out, screw the tool down to push that little piece of scrap through. You can then give the tool a little shake to ensure that the metal falls right out.