So let's take a look at our horse. To begin with, look at the overall space the horse takes up and consider its proportions. This will vary from horse to horse, and will also change depending on your viewpoint (where you are standing in relation to the horse) or the angle the photograph was taken, if you're using a photographic reference.
There are no set rules about how you should divide up the space - try experimenting and see what suits your way of working. I tend to look for clearly visible points - in this case, I've drawn a line roughly across the withers and croup so I can see how the shoulders sit in relation to the hindquarters. Add a horizontal under the barrel and I can see how deep the horse's chest is an the proportion of the body to the legs.
Dropping a vertical through the withers or a little higher helps you to see the 'weight' of forequarters. Also think about the angle of the neck. A lot of this stage can happen in your mind, before you even put pen to paper, but you can also sketch these lines lightly if you wish. In the next step we'll continue looking at our subject before we start sketching.