Firstly we must begin this tutorial by using a picture (Preferably a face).
I am going to use this picture of David Beckham:

Throughout this tutorial we're only going to use one tool.
This is going to be the liquify tool.
We simply open up our picture then go to: Filter> Liquify...

This tool is quite basic and very easy to use.
For now we are going to leave all the setting except 'Brush Size' and 'Brush Pressure' to their default settings.

It is overall very simple to use the liquify tool, simply take your cursor and drag the picture in any direction you wish to pull your picture.
Try experimenting with the brush size and pressure as it effect the effort used and the quality of what you are doing.

Tips:
When distorting a face here are some simple tips that I will provide.
For effect I recommend that you enlarge or decrease the following facial objects in size:
Eyes, Forehead, Nose, Ears, Mouth (If open), Chin, Nostrils.

I suggest that you just simply tweak (stretch, squash, flick) the following facial features:
Eyebrows, Hair, Mouth (If closed), Facial Hair, Neck.

I also recommend that you simply experiment, as much you want to give you an experience with this tool.


This is what I did to the David Beckham picture:

Before	After