This is actually cumulative throughout your quick matches. Just aim a little bit at the head when shooting. A great gun to get this is actually your starting weapon which is a shotgun. You will know it is a headshot by two different indicators. One is on the right where it shows you what gun you killed them with. Next to that is a special icon showing if it was a headshot. The other way is to look at the left of your screen where it shows your points for kills. There will be a 15 point headshot bonus one there. To check your total, just go to Player Status on the menu where you start matches. This will likely be something you gain without trying. They do not have to be "kill" shots, regular headshots count too.