This is the photo I chose. A couple of reasons I went with this picture over a few others are that I like the lighting scheme and that there are a few textures here that I've never painted before.
First off, the lighting is coming in at an angle, which is great for reading forms. There isn't much point to doing a study if you gain no insight into how light works from looking at the picture, and let it be known that the key to realism is lighting. Also, notice how the light is at an angle and the direction of the face is at an angle, but those angles don't line up. That's a good thing. Nothing is directly aimed at us, so we get to see how the light wraps around the shapes very well.
As for these textures...I knew I didn't want to use photo textures, so choosing an image with little pattern-like textures was a tough call. In the end, I knew I could use the challenge, so I went with it.
Here's my framework sketch that I began with.
Actually, I wouldn't even have this process picture if I didn't completely mess up when I started. At first I wasn't going to do a clean sketch; I just did some quick lines to get a composition. When I started to paint I realized how awfully placed my marks were so I went back a step as not to waste my time working with something fundamentally flawed.
That's another thing to note when doing studies; they're studies. You aren't creating a work of original art, so there is no rush. One of the biggest mistakes you can make from doing studies is to notice something is wrong and to leave it wrong. Since what you're making is a study, that moment is a perfect chance to correct what you mess up, which in turn ingrains into your head how to fix those problems. It's the essence of a study. So, next time you want to do a study, take your time, look for your own mistakes and fix them.
Anyway, so as you can see I cropped the image to showcase the face. I also changed the direction of the stare. I wanted my image to capture a sense of hopefulness. The original image had a slight sense of that, but I don't think what I wanted was the intended message in the photo. For the real thing, it gives off more of a sense of this thoughtful down-to-earth girl in contemplation, while the outfit sets the style. The way I wanted it was a more awe-inspiring approach. So for that I made her look straightforward and above us, as if she is looking to the skies. With that, the outfit seems to make more sense. Like she's longing to fly.
At this point I try not to get caught up in blending and smoothing everything out. This stage is all about color/value accuracy. If I find a color that I think looks right I'll blot it in. If it holds up to the colors surrounding it, I move to a different area. It's a bad habit to start blending in your colors right when you put them down, without placing the rest of your colors. The thing about color is that the way we see it is largely relative to the surrounding colors. It's why a grey against orange looks blue. If you just start dabbing blue everywhere you may find your colors to be t0o saturated/desaturated/warm/cool/etc. Your best bet is to work large and to not get bogged down by all those details.
Also note that I started with color right away. Deciding to start in color or black and white is very dependent on how comfortable with values you are as an artist. When painting there is no "just color," you either do a painting with just values or a painting with color AND values. Color has it's own inherent value. Either way, you need to make sure your values are good, so if you think you're going to have some trouble, don't think twice and just do it in black and white. Jumping into the deep end right away is a method of learning, but you'll end up wading through all the confusing bits, which not only takes longer but can be very discouraging.
That's another note to take, if you do a study, especially in color, don't color pick from the photo. I say especially in color because, like I said, the colors have inherent value. You're color picking the colors and the values. At that point, what are you learning? How are you challenging yourself? And in the end, will it feel worth your time and are you a better artist because of that study?
Also, if you're starting color, keep the lighting in mind at all times. Knowing how the light affects the colors is very important. Remember, lighting is key to realism. Here we have warm light from the sun. What warm light will do is saturate warm colors and desaturate cool colors. Also, since the light is warm, the shadows will be cooler. Anyway, time to move on.
After I set up all I wanted to under my lines, made a new layer over them and got to work rendering. Here's the long grind, where you can take all the time you need smoothing things out and detailing all you want. As long as your framework is good, go at it. That said, you may often find that your framework wasn't perfect. During this stage a few times I found myself using liquify and warp and such to adjust angles that felt wrong, or change the sizes of things (also just repainting areas). This is due to my shortcomings in my knowledge of skull structure, which means I need to study those more too. Anyway, with this shot you can still see a few lines as I work around the image. I like to keep my lines layer full opacity, while I work over them. I know some people like to lower the opacity or mask out or turn off the layer completely, but the way I see it is that alone they may be lines, but when you start filling up your image with color and value, they become part of that. Also, what you are creating is art, not a simple reproduction. If a line looks great in that one corner, think about keeping it there. What we make is poetry, not a research article or a page from a dictionary.
Okay, so after more painting into my lines, correcting some of my drawing mistakes, painting in those textures, etc, here's the final image. There's always more I could do but I think I hit a good cut off point. I wasn't keeping track of the exact hours, but it was on and off for about three days, if that helps. I hope you guys learned a little bit. Now go and study something!
(If you do an image search for Catherine Bell, you'll find a few good portraits like this one, for future reference.)