Hello!
Welcome to Daxx's 2nd Quickie-tutorial called My tips on getting a Decap
This method is effective in both multiplayer and singleplayer.

Basically, the most efficient way of getting a decap is aiming your (extended) foot and Kicking RIGHT where Uke's neck and throat is
Example:

Notice that It looks like (and is) that My foot is going through *censored* name's head. this would be a perfect decap if there would happen to be a dismember threshold.
A decap by doing the same thing would look like this

(No I am NOT Doing the Hampa headkick)
I hope this tutorial helps you
Cheers,
-Daxx