I is important to note it is generally a technique that isn't looked favourably on by singing teachers because it calls for you to put a lot of stress on your vocal chords. So please use it sparingly.
A quick way to understand how to create the sound is to try and attempt to growl like a dog. You should feel a vibration at the top and back of your throat. Once you can do that you can then apply that same technique to words and ultimately incorporate it into singing.
Below is a video tutorial explaining, in more depth, how to create that vocal styling. (Although the tutorial below is applying to the Death metal genre it is still the same technique used by singers like Christina Aguilera).