Find suitable models of horse heads. Horses can position themselves in many different poses, so take time to look them over and pick a few different shots to use as guides for your sculpture.
Visit websites like The Sculpted Horse and Equine World for more ideas, photos and examples to follow. (See links in Resource section.)
Organize the different pictures according to aspects of the horse's head. For example, maybe one picture is not the pose you want, but it includes a great close up of the horse's eyes. Keep it, because you will need it as a guide for eyes later.
Sculpt a foundation of the head, whether you are carving an entire horse statue or just the head. This is the beginning layer or a rough outline of what your finished product will be. This is where you decide the general pose and position of the horse head.
Tighten your sculpture's focus by carving away more rock. Begin making the general outline of where the nostrils, eyes, mouth and ears will be. Take great care in making these choices, as it will affect the overall balance of your sculpture.
Provide details on the sculpture. Add definition to the eye, nostrils, mouth, ears and rest of the sculpture. This is the step that will provide character and realism to your sculpture. When you are finished, your horse head sculpture will have greater character and depth because of your efforts on this step.