Let me first remind you how important it is to start with a clear photo, of which really accentuates the eyes of your pet. Once I have painted the eyes, and in so doing, be cautious to really SEE what you are looking at. Color is essential. I never use black from a tube. It is "dead". Rather, I blend ultramarine and umber until I have a deep blue black. The pupil is the darkest in all eyes. The iris varies in values depending upon the light source, and the outer rim of the iris is very dark. Look closer, you'll see. Never forget to add a bright sparkle of white to indicate life.
When I know I am comfortable with the eyes, I work the face in, using various colors and values. Look closely at the animals nose. Study what it does. Study the colors!!! Look even closer!!!! It's amazing all of the different shades of color you can find - then use them!!! Next, start working the portrait thoroughly rather than concentrating on one section and another. Work it all together. Go back to it again and again. Turn it upside down to make sure your proportions are in order! Have fun!!!
Feel free to contact me if you have any questions.