I used to use pencil to outline all of my art on canvas before painting but it would become a huge mess when I wanted to make a change, and I would always pickup the wrong pencil that left very dark lines. So I've been changing the way to approach a painting and I've switched to using charcoal.
Initially I will prepare a canvas by using gesso regardless of whether the canvas has it already. I don't like a perfect surface, so by doing this it makes the artwork feel like it has already started. I then start the next stage in two different ways. I will either have a reference image I use as inspiration and draw directly onto the canvas. Or I will spend time in photoshop mocking up a reference image (which is a mashup of lots of different imagery), print it out, and then transfer it by eye to the canvas surface.
Either way I will use charcoal lightly to draw out the scene, as you can see above. Then as I paint I will choose a section at a time and block in colours before detailing each area one by one. I always focus on the figure or main object first before the background or inputting the final details. I see my figures as a character in my mind, so the quicker they have an identity, the easier I find to finish the work.