In call of duty 4?

I guess to get all headshots.

Are you asking for tips on how to get headshots or are you asking how is it done?

 For tips to get headshots.

The best tip is to aim high, start at around the high chest and the recoil will place it on the head, with most guns.  Use the red dot sight, i like stopping power maybe use steady aim or deep impact too.  I'm not sure how often you play but getting the golden AK is very hard to get.  I have most of my Rifles complete, the G3 and the MP44 are proving hard to get for me.  The M4 and the g36c are pretty easy to get.  The M14 go straight for the headshot, 1-2 shots will get it, with stopping power.  The M16 is another one where one  burst will kill to the head.  With the Ak i went with the silencer approach, and tried to sneak around since the recoil is pretty brutal on that gun.

Try not to run around the map when going for headshots, find a nice tactical position, somewhere defensive, around the middle of the group. And play domination, its pretty easy to get headshots when they are trying to capture the positions.

Aim for the head. That helps

Aim for the chest, then swing up. Your opponent's head is usually somewhere in that area.

Well the EASIEST way is to join a cage match and ask the other guy if he wants to just get headshots.

but if your planning to get them legitimately, then do what griffin said.

hack the game

Go to the cage match lobby, and before it finds you a game, invite a friend who wants to do the same.

Now just sit back, and let the headshots come rolling through. If you do it right, it won't have an impact on your KD, either.