Here I'll show a process of a portrait inspired by classical oil paintings. Some of my greatest inspirations are the works of Sargent, Boldini, and other 19th century painters, and here I'll try to show some of the principles they worked with.
I used this brush: https://assets.clip-studio.com/en-us/detail?id=1701386 https://assets.clip-studio.com/en-us/detail?id=1701386
It's a very bold brush, therefore very suitable for "painterly" work where you focus on big, strong shapes and aim to make every brushstroke count.
For the same reasons traditional oil painters recommend a big brush, this one is ideal for beginners who want to learn how to simplify and pay attention to the big picture.
1) I start out with loosely referencing from an old photo of mine, traced directly.
Some artists will ask: is tracing okay or not? The answer is: it depends on your goal. If you want to train your eye and accuracy, I recommend copying instead of tracing, to force yourself to see the shapes and proportions quickly and accurately. But if you only need to get a sketch or lineart as quickly as possible, as professionals usually do, we usually trace.
Note that a reference photo is almost never "correct". Photographic lenses distort proportions and perspective. Here, the foreshortened arm and hand look too small when traced, so I enlarged them in the drawing.
In a photo, our eyes accept such distortions because we're trained to accept a photo as "truth" no matter what, but the distortions won't look good in a drawing. So don't trust your reference fully, and always be prepared to make adjustments in the drawing!
2) In the manner of the oil painters of old, the next step is a background underneath the drawing.
I indicate it with some loose brushwork, we'll get back to it later.
3) Now I block in the shapes of the figure, with a still large brush.
Note how all this paint goes on a new layer over the sketch layer. The sketch will be slowly painted over, and lightened with the eraser from time to time.
This might not be traditional, but I use the airbrush tool in digital painting a lot - here, to create "lost edges". Of course there's a bit too much softness at this stage - we'll go over it later.
5) Now I paint the dress.
My goal is to indicate its 3dimensional shape and folds with using as few brushstrokes as possible.
Don't forget to mirror your painting from time to time to check it with "fresh eyes"!
I also like to have a smaller, zoomed-out version of the entire work next to me at all times. This helps me keep track of the whole, even when I work on details and small areas. In my opinion it's very important not to lose sight of the whole work.
6) Next I start painting the face and hands.
Here, too, I use airbrush to hint at a blush and reddish areas. The airbrush is great for this because it can be very subtle and soft. Use a strong color with a very light touch!
The head and hands will get the most detail, because they are our focus points. Here I model the head, make its shape 3dimensional, and add detail. The hair receives the same treatment. Brushstrokes will stay visible, since we're aiming for a painterly look.
Also note that I'm not copying closely from the reference, I use it loosely at this point because an exact likeness is not needed.
Adding more detail to the hands and face:
Finally I go over the background again, experimenting with shapes and values until I have something that I'm happy with. I also add highlights to the hair, and refine some edges here and there.