On the right is the 8 x 10" panel I'll be painting on. What you're seeing is the first stage. I've applied a one-inch-square grid to the print and a corresponding grid to my panel. The grid system dates back at least to the Renaissance, and is a great way of getting accurate proportions down.
You'll notice that the photograph "cut off" the feet of the horse, and that I've compensated for that. I allowed extra room at the bottom, and then sketched in the hooves before I did the drawing on the panel.
Not obvious yet: I will simplify the background of this image a LOT. All those busy, busy tree trunks? They're not helping the picture, so most of them are going to go away, and the ones I do keep will be soft-edged and their colors greyed down so that the focus is where it should be, on the people and the horse.
For the drawing, I'm using pastel pencil, which is delicate and will simply vanish into the oil paint. Anyone who's ever tried painting over standard graphite pencil lines will know why I don't use graphite! It bleeds through like mad and the grey color tends to muddy up the paints.
If you have questions about any of this, do ask. Most people never get to see a painting in progress and it's fun for me to talk about how it works.