VERY VERY carefully - always start by taking off mess than you think! If you aren't sure then you should get a hairdresser to do it for you - and pay attention to how they do it so you can do it yourself next time.
Wet your fringe. use a comb and if you can get hairdressing sissors (they give a better cut) stand in front of a mirror and slowly cut across your grown fringe. use the comb as a barrier to stop you cutting too much away. if in doubt...Wait for a hairdresser!
Be careful if you do cut it wet!! If you cut it wet when your hair dries it will be shorter, so if you cut it wet leave plenty of length on it, I always find dry cutting is easier as you know exactly how it will look when its dry, obviously!!!
You should cut it dry so you can see how long it will be when dry. Comb it down straight over your eyes and cut in little snips holding the scissors vertically. Go for longer than you'd like with the first round of cutting--you can always cut more but it will take a little while to grow out. Just don't cut across in a straight line.