Once you can handle watercolor materials, you will probably want to make some paintings. This might seem obvious from the beginning, but it is something I discourage in new painters until they are proficient in the media.
There are so many other elements of making a painting besides being able to handle the materials. Like putting on a big dinner party, knowing how to cook is only one of the elements needed: you need to have timing, judgment about menu, decorations, good guest choices, knowledge of wines, and possibly a maid for cleanup.
For a painting, you might think subject is a key concern. But really, a good painter can paint any subject and turn it into a good painting: dead animals, parking lots, the back of someone’s head, a puddle, or a rock: all subjects painted by experts resulting in world-class art.
Most people settle on the standards to begin with, but only because it makes them feel more comfortable to do something that has been done before, such as a sunset or pretty child. There is a faint hope in the painter that this will make the finished product more acceptable to others, and it will. Later the painters will think less about the “average” viewer, and more about the treatment of the subject
Loving the thing you are painting can be a detriment to doing a good job on the painting. You might think too much about how much you love a certain kind of flower, or a grandchild, to concentrate on the more objective thoughts you should be having while you paint.
Composition is probably the most important consideration in making a painting. Like it or not, most paintings have four straight sides, and the relationship of the data inside to those lines is far more important than first realized. Witness the difference between the average snapshot a person takes with no regard for this idea, and a professionally framed subject showing thought for how material fits into the rectangle.
Understanding good composition relies on knowing about values and line. You have to see the subject as an abstract set of intersecting lines, along with graduating values within the colors, and be able to make adjustments to cause the lines and energy of the painting flow. To do this you have to ignore what it is you are painting: an eye, an apple, a tree. They must become line, value, and color composites that you arrange to suit the overall composition. This can be done by considering many possible arrangements of the materials you are painting, or many possible framings and lighting situations.
There are “rules” like the one-third rule, odd numbers of items, and variety of volume in shapes, but its best to rely on your intuition. The most common mistake is not considering composition at all, or enough, and hoping color and appeal of subject matter is enough. It is not.
Scale is another important consideration. For impact, you might