Shading an animal right is what makes the whole artwork look like it was done by a pro. When I started drawing animals I focused to much on painting the animal right instead of looking at the proportions of my animal drawing. So before you start shade and paint be sure that you are satisfied with the animals lines so far, because once you start paint and shade it will be very hard to change any lines of your animal drawings. So if you are absolutely sure, then let's start shading the animal.
One big tip here is that you start at the upper left corner (or upper right corner if you are left-handed). This is because it reduces the risk of the animal drawings to get all messy. So start from the upper corner! Think about how you want the whole painting to look, try to get an image of it, then you start slowly adding with your pencil this to the background. If you need reference photos then go a head and get those, these will help you not to mess up to much on the background proportions. Don't start with the darkest area, instead start coloring the lighter area of the background of you animal drawings, and then go to the darker areas gradually. Then when the upper corner is done, then start moving the center area of the animal painting and continue with that area and then next and so on.
A trick for making the animal look very well shaded is to exaggerate the contrast. Where you want it to be black, be sure to make it black and not some "grayish-black", this is a little trick lots of gifted artist miss out on. Same goes with the different shades on the animal, going from dark to light, be sure to really make the dark areas dark, and the light areas light.
And remember to take some breaks from your animal drawings, don't rush it and try to get it all done within an hour or so. Take a break after an hour or so and then come back later and you will probably spot some small mistakes that you didn't see while you were doing them! And my last tip is the most important one: Be sure to have fun while drawing!