I thought I'd post a little art tutorial to show a step-by-step approach to drawing portraits. I used the photo VALентина by Ирина-Марьенко as reference, as a personal (non-commercial) exercise.
Starting off. Sometimes I draw a light, loose oval to rough in the head size, but my Portrait Drawing & Painting class teacher taught me the benefit of using straight lines to block stuff in. Straight lines are easier to adjust and leave you with more blank space, but your brain can still fill in the lines so it's not too confusing.
So, I used these initial lines to block in the rough size of the head, hair/hairline, face area, and shoulders.
I tend to focus most on the face when starting portraits out since that's generally the most important area. Still using straight lines here to add more angles and indicate the placement of the facial features. I've had the best success with getting the overall shapes blocked in before getting caught up in the details.
Refining things a little more, getting the eye socket area blocked in. (It kind of helps me to get that area blocked in first--especially when the eyes are in shadow--and then worry about dividing the space to fit the actual eyes in. I also roughed in the edges of the face.
In my drawing I ended up unintentionally straightening the girl's facial features so her head wasn't as tilted and her right eye (the viewer's left) was lower than in the reference photo. It's not all that noticeable at this point in the drawing, but I'm pointing it out now because this would have been a good point to stop and double-check my angles (possibly using a ruler to compare the tilt in the original photo with the tilt in my drawing) in order to prevent myself from making that mistake.
This is also a good point in the drawing to double-check the proportions in general and make sure you're on the right track. Mirrors come in really handy for drawing portraits (use the mirror to compare the original photo with the drawing). Turning the drawing upside down also helps. I wasn't getting too picky with this because it was just a practice drawing, but that's what I would do to spare myself some later heartache when working on a portrait that I wanted to really get right. It's easy to correct things at this stage (though it's hard to spot what's going wrong), whereas later it becomes more clear what's going on wrong and far more difficult to fix it, when you've got more complicated details, possibly shading, and a lot more time invested in the project.
Getting the eyes, nose, and larger sections of hair more refined, plus roughing in the scarf. I could get more accurate with the cloth folds in the scarf, but it's not as important to me as getting the face and hair right.
Getting a lot more details in the eyes and blocking in some basic shadows. The lines I added in the hair are to indicate either light or dark areas. This takes more discipline than I usually have, but makes things look neater later on, versus taking the alternative route and freehanding the shading and hoping I get the placing right.
More work on the hair (see how I filled in those marked-off areas?).
Some light shading in the scarf and hair (I compared them to how light the girl's face is in order to get a base value; if I wanted some lighter areas in the scarf or hair later I could "draw" in the highlights with an eraser).
Finally blocking in the lips more (I still feel uncertain when drawing lips so I tend to procrastinate on them).
One thing to note when shading edges: generally speaking, the areas in the light will have harder edges and more contrast than areas in shadow. The side of the girl's face that's in the light had a much harder edge than the side that's in shadow, and same thing with her hair. In the area in the shadow, you can still tell roughly where her face stops and her hair begins, but the edge is a lot hazier. Our brains tend to want to "translate" what's happening here, and since we know her face and her hair are two separate entities, it's easy to make that line hard without realizing it really isn't. This is just a suggestion (and I don't think any rule/tip about drawing fits in every circumstance) but when I have a hard time telling where one thing stops and another thing begins in a reference photo, I've learned to try to copy that in my drawing rather than interpret the line for the viewer. It actually makes things a lot easier for me.
At this point in the drawing, it's getting more obvious when comparing the drawing to the reference photo that I got the tilt wrong. Also, her eyes are too close together in the drawing and the shape of her chin is too abrupt (too light and jutting out too far).
Finishing off with drawing. For an exercise, I was content to end it here. I would do a lot more correcting and cleaning up (especially with those smudges on the right side!) if this were a serious project. In that sense, this isn't nearly as useful a walk-through as using one of my real portraits as an example would be, but hopefully it still gives you some good starter points for going about drawing a semi-realistic portrait.
Again, here is the link to the original photo if you want to compare the two (*winces in anticipation of potential judgment*).
And yes, I did use a Papermate Classic pencil to draw this, despite owning a set of perfectly good drawing pencils. I'm a rebel at heart.