There are a variety of ways in which to approach creating a painting, none of which is better or more correct than another. Which approach you take will, to some extent, be influenced by your painting style and personality.
As with all painting techniques, don't assume a particular approach won't work for you without having tried it. Nor do you have to use only one in a painting, you're free to mix and match approaches if you wish.
With a blocking-in first approach, the whole of the canvas is painted or worked up simultaneously. The first step is to decide what the dominant colors and tones are and to loosely paint these areas, or block them in. Then gradually the shapes and colors are refined, more detail added, and tones finalized.
Blocking in is my favorite method of painting, as I rarely plan a painting in great detail before I start. Instead, I start with a broad idea or composition and refine it as I'm painting.
Blocking in makes it easy to adjust a composition without feeling I'm covering up or changing anything that's so beautifully painted I can't lose it.
Some artists like to approach a painting one section a time, only moving onto another part of the painting when this is finished. Some gradually work from one corner outwards, finalizing a certain percentage or area of the canvas at a time. Others paint individual elements in the painting, for example, each item in a still life, one at a time. If you're using acrylics and want to blend colors, it's worth trying.
This is an approach I use very rarely, but find useful when I know that I want to let part of the foreground in a painting intrude into the background, such as waves dashing up a sea cliff, when I don't want to have to try to fit the background in around the foreground right at the end.
Some painters like to start with the detail, working up these areas to the finished state before painting the background. Some like to get half or three-quarters of the way with the detail and then add the background.
This is not an approach to use if you're uncertain of your brush control and worried you're going to paint over something when you add the background. Having a background that goes around a subject, or not quite up to it, will ruin a painting.
Tina Jones, whose painting Faces of Karen Hill is shown here, adds the background when she's at about the halfway mark. After adding the background, she then made the colors of the skin and clothing darker and richer, refined the overall shapes, and finally added hair.
If you paint the background first, it's done, and you don't have to worry about it. Nor stress trying to paint it up to your subject but not over it. But doing so means you need to have planned it out, visualized the colors in it and how these fit with the subject of the painting. Not that you can't change it later on the painting, of course.
Some painters like to do a detailed drawing first, and only once they're totally satisfied with this do they reach for their paints. You can either do it on a sheet of paper and then transfer it to the canvas, or do it directly on the canvas. There is a strong argument to be made for the fact that if you can't get the drawing right, your painting will never work. But it's an approach not everyone enjoys.
Remember a paintbrush is not simply a tool for coloring in shapes, but that the direction of the brush marks will influence the result. Even if you feel as if you're coloring in a drawing, it's not the kind that a five-year-old will do.
This is an approach that requires patience and is not for anyone who's in a rush to get a painting finished or to get the colors sorted. Instead, it involves first creating a monochrome version of the painting that is as finished as the final painting will be, then glazing color over this. For it to work, you need to glaze with transparent colors, not opaque. Otherwise, the form or definition created by light and dark tones of the underpainting will be lost.
Depending on what you use for the underpainting, it can be called different things. Grisaille = grays or browns. Verdaccio = green-grays. Imprimatura = transparent underpainting.
Alla prima is a style or approach to painting where the painting is finished in one session, working
instead of waiting for the paint to dry and building up colors by glazing. Quite how long a painting session depends on the individual, but the limited time to complete the painting tends to encourage a looser style and decisiveness (and the use of smaller canvases!).