I personally don’t divide the paper into sections like you did, but if it works for you, do it.
I’ll write out the process I did for the Arbiter Portrait I drew and posted on here.
Basically, I started off staring at a reference (in my case, a thumbnail of the Arbiter in the Halo 3 Guide). After looking over the reference for a good 5 or so minutes, I removed the reference from my view when I started the actual drawing. This is because I can remember the basic shape and location of key parts of the Arbiter’s face and helmet while not getting caught up in tiny details - yet. It is important to focus on the basic shape first, otherwise you’ll detail areas separately, and in the end it probably won’t “read” as a drawing.
For the actual drawing, I did a line-drawing of the entire helmet, and edited it around until I thought it looked right - it is important to make any major edits before you start shading. Then I started shading in the darkest parts of the subject. I start dark to light because I primarily use 2HB Pencils, which means you can always go lighter, but eventually you can’t get any darker with 2HB. After that I shade the entire rest of the drawing with the same darkness, after which I slightly erase the lightest areas.
Once the overall shading is done, then I look back at the reference to look for any small details I missed. In my case these small details were the teeth and wrinkles on the Arbiter’s face.
That was my basic process for my Arbiter Portrait. Using this prioritization, as well as the knowledge gained from my hundreds of previous drawings, the drawing was finished in under 45 minutes.
If you want a video process, I have a tutorial on how to draw an AH-1z Viper on my Youtube Channel, which is comprised of scanned images from various points in making the drawing.
The video can be found here
AH-1z Viper Drawing Tutorial
Note: I drew tons upon tons of profile view subjects before I was 12, upon which I started understanding shadows and light. I’m only 15 now, so it only took about 3 years for me to understand shading. It really doesn’t take as long to improve as some people may think.