The leather texture is going to be the only problem
not really seeing as the leather texture has been the last thing you should have been concerning yourself with this entire time.
This is what you should be doing...

I will go through the steps, this is in no way about the texture, its about forms, volume and lighting (texture is to small a scale to be dealing with at this stage).
Step 1: Create the shape of the object. A silhouette of the object is all that you need to begin.
Step 2: Highlight the areas that would be being hit by any light at all. (This tutorial is very sloppy so it misses a number of areas.)
Step 3: Aren't some areas hit by more light then that? Yes, then highlight these areas more, the top of cheeks, the brow, the nose, the forehead and chin are all hit by more light then the rest of the face as the light is coming from the top.
Step 4: Repeating step 3 essentially!!!
This little sum up of how to portray depth through your shading misses a lot but is simple and hopefully you can take something from it. When drawing something you are giving an illusion of an image. An image itself is made up solely of light being deflected back at you in different ways, or not being deflected back at you. Light performs a lot more actions then the ones i detailed here and so this is a very basic outlook at how you should be shading.
So what is texture in an graphical sense? texture is the small scale material shapes and forms that change the manner the light reflects, and hence the way we perceive the image. If you just draw a texture you just draw a flat material, you have to think of the object without a texture first so that you can make it look 3 dimensional.
***EDIT***
Just thought I would let you know I am really just recanting what everyone has said before, and what a number of people have said in critique of your other pieces. I am just trying to say it in a way that is hopefully more understandable and 'step by step' for your benefit.