I always start out with a photo from the client. Preferably, the photo will have complimentary lighting on the subject, along with good contrast between the light and dark areas of the image. Higher-resolution photos are ideal - since I am referencing this static image to capture the details and personality of the horse, the more visual information available, the better. It is also best to have an unobstructed view of the horse's head, but in most cases, I can remove tack for a "wild and free" look fairly easily. Keeping the tack as part of the portrait adds an additional level of complexity, but if the horse's face is beautifully framed by the tack, I will consult with the client about keeping it in the artwork. Basically, I remain as true as possible to the photo, and only add small enhancements such as trimmed whiskers and ears, and highlights to the eyes.
This photo of Freckles exhibits everything I look for in a photo to work from:
I am very pleased with how Freckles' portrait came out, as I hadn't drawn a multi-cloured horse wearing tack in quite some time. I hope you enjoyed seeing my process in creating hand-drawn artwork from photographic reference!