(You can make your own transfer paper by coating a sheet of thin paper with graphite.)
Place the graphite side down on the drawing paper, put the line drawing on top, and transfer the lines by going over them with a sharp lead.
Don't press too hard or you'll dent the paper. Just press hard enough to get a clear transferred line.
The First Fill Layer
Put some finely ground graphite on a scrap piece of paper. Dip a small brush into it, make some practice strokes, and then use it to fill in the horse's face.
Be careful not to rub out the guide lines when you're doing this fill layer.
Fill in his lips, face and ears. Don't try to darken any area to its full value yet. Instead, make this first layer a little lighter in all areas than what you see on the finished piece. Leave his blaze and eyes untouched for now.
If your brush marks look streaky, you can help smooth them out by going back over them at right angles using a scrubbing motion.
Fill in his mane and neck next. Fill his mane into about a 2 or 3 value for now. To create a 2 to 3 value with your bush, make sure its almost empty on a scrap piece of paper first. His neck is very dark so you can really scrub in the graphite there with the ends of the bristles. Start to notice where the shadows are falling on his neck and chest. The darker area goes all the way down to the bottom of his chest and it's at an angle.
Use a kneaded eraser to clean up any graphite that has strayed out of the lines.
Nostril Details
Create some ground graphite, dip a paper stump into it, and make a few practice strokes on a scrap piece of paper. Using the tip of the stump, work some of the darker details into the nostrils, lips and sides of the face. Notice that the lips get lighter towards the edge very gradually.
The nostrils have both hard and soft edges you need to pay attention to. The insides of both, near his blaze, have a hard line and the outer edges softly lighten out to the edge. You can use a stump to pull the graphite out into the soft edges.
Notice that his bottom lip is visible on the right side and it's very dark. Use a 2h pencil to darken in that area. Again, don't darken anything in to its full value yet.
Face Between Eye and Nostrils
To darken the right side of his jaw, use a 4h lead to do a fill and your sharp stump tip to fill in missed spots. Use a brush to help you smooth if you want to.
Notice that I've stroked in some dark detail lines on the left jaw and darkened in the dark detail under his eyes also. Follow your detail lines to get these areas positioned right. Use a stump or a 4h lead to do this.
Ears
The ears are a very dark 81/2 to 9 value. Don't darken the ears in to their true 9 value right yet. Use a 2h and carefully fill in around the edges, highlights details, and mane hair. Don't press too hard with your lead.
Smooth your marks with a stump but be careful not to lose the hair detail. Pull some graphite into the ear highlights with the stump to darken them.
Neck and Chest
Near his face, his neck is an 8 or 9 value and gets lighter to the left. Use a sharp 2h lead to darken his neck along his jaw line.
Leave the general area of his mane untouched for now but continue darkening his neck down into his chest. The 2h lead with leave a streaky fill that you can smooth out with a brush if you haven't pressed to hard. You can also use a stump to help to smooth and it will also help darken the area a little more.
The muscles in the bottom of his neck are about a 5 or 6 value. Define the shadowed areas of the muscles first with a 4h lead first, so you won't lose them. Pull the graphite over into the lighter areas with a stump or brush to make a gradual transition. Use a 5h lead to darken if needed.
Use a kneaded eraser to clean up any smudges outside of the horse's outline.
Transfer the line drawing to the paper.
Put some finely ground graphite on a scrap piece of paper. Dip a small brush into it, make some practice strokes, and then use it to fill in the horse's face.
Create some ground graphite, dip a paper stump into it, and make a few practice strokes on a scrap piece of paper. Using the tip of the stump, work some of the darker details into the nostrils, lips and sides of the face. Notice that the lips get lighter towards the edge very gradually. 
To darken the right side of his jaw, use a 4h lead to do a fill and your sharp stump tip to fill in missed spots. Use a brush to help you smooth if you want to.
The ears are a very dark 81/2 to 9 value. Don't darken the ears in to their true 9 value right yet. Use a 2h and carefully fill in around the edges, highlights details, and mane hair. Don't press too hard with your lead.
Near his face, his neck is an 8 or 9 value and gets lighter to the left. Use a sharp 2h lead to darken his neck along his jaw line. 
