The first thing you have to do is go into Cat Fights, which is under Events and above The Mall when you are at the main Girl Wars Online Home screen. Find a person to fight here that you are convinced you can beat. This means that they have a mob count that is less than that allowed by their level.
If you are above that limit, you can almost guarantee that you will win every fight with that Girl Wars Online player. Select to View that Girl Wars Online player profile, and once there begin attacking them. Do this until they are in the Hospital, and you are not able to attack them anymore. After this, slap them again once or twice to ensure that they have a very low life.
Now look to the right of the Fight button and hit Add to Bad Girls. Bad Girls Dissed, which is really like a bounty list, is where they will appear once they have received the bounty. Select Bad Girls from the lower task bar in between Invite and The Diva. Once here find the Girl Wars Online player that you put on the Bad Girls menu and attack them.
At this point you will probably only need to attack once before killing them. You then also receive most of the bounty that you placed on them. Though you cannot attack a Girl Wars Online player who is at The Spa normally, you can if they are a Bad Girl Dissed.