Though some works are completely unplanned and just happen, usually the artist sits down with a vision of what they want to create. Planning a work of art can be a help as you know exactly what you are going to do, but it could also inhibit spontaneity.
Letting a painting evolve as you work is very free and lets you be spontaneous, but also leaves you open to the possibility that the work of art will not go anywhere and you will end up with a mess. Ultimately the degree to which you plan out a painting depends on your personality.
Some artists find it necessary while others find it impairs their abilities. No matter how detailed you like to plan, there are several decisions that have to be made before you to start to paint.
Deciding on a subject is the logical first step as it influences the format of the support, the type of support used, and the technique you are going to use to create the painting. If you have only a vague idea of what to do with an appealing subject, like a beautiful mountain landscape, sketching or doing small evaluations rather than a full painting will enable you to see whether the composition and selection of elements works well without wasting time or materials.
A pleasing evaluation can then be used as the basis or reference for a full-scale work of art. On the other hand, if you find that doing a small evaluation makes you stiffen up when you come to do the large-scale paintings, consider doing only quick sketches to see if a composition works and taking reference photos to work from back in your studio.
Some people do not like starting small because you become so focused on replicating it, rather than it reminding you sufficiently of the original scene, so you can complete the piece.
After you have decided on a subject, you need to decide what the best format for the support is.
You need to decide whether the orientation of the canvas should be landscape, portrait, or square. You should also decide what shape of the canvas will best suit the subject matter.
For example, a very long and thin canvas used in adds a sense of drama to a landscape, especially one of a wide-open space. The size the support will be should also be a conscious decision.
A painting should not be a particular size simply because that is the size of the sheet of paper you have. If you buy primed and stretched canvases, have several in various sizes to hand so you have a choice on different occasions.
Think about how the subject would look if it were painted small, or perhaps very large. Consider the effect the size will have on the message; for example, portraits which are oversized are very dramatic.
If you only ever use one medium then you do not have to decide which one you think is best for this particular subject. You also need to know what technique you are going to use.
For example, if you use acrylics, you can use them thickly or thinly, like watercolors; and you can use retarders to slow down the drying time. If you use watercolors, decide if you are going to use masking fluid to keep areas white.
You also need to determine the type of support you will use. You can paint on canvas, primed hardboard, or paper.
There are also different varieties of canvas; like a fine weave, such as linen, or a coarse weave that will show through. If you use paper it can be a smooth, hot-pressed paper or a rougher watercolor paper.
This important decision will not only influence the texture of the final work, but also how you work. If you are using oils, acrylics, or gouache, know if you will you be using a ground and what color it will be.