Hi! Made an edit for you.
So, while fox fur is pretty short, some parts of their fur is nice and fluffy, so I only rendered those parts as fur and the rest I left pretty simple.
Picked top left as my light source.
Fluffy parts: tail! the lighter bits under the chin and down the chest, and the cheeks.
I also made your foxy look more strapping by reducing the stomach bulge (and shading the underside darker).
I bend the rear legs and gave the front legs a more active pose.
I also moved the cheek puff and the ears.
A lot of my changes are just personal choice. You are welcome to continue your style.
Things that you need to change are the lighter parts that come across the fox's front right leg (which I assume is in front due to the shading), and work with the light on your tail a little more.
As for how to shade fur. As you can see I've just made triangles facing downwards and to the back of the fox of the lighter colour in each case. sometimes this creates shade on the hair beneath. I also added more triangles to the silhouette to imply fur further.