There are both and pros and cons to planning your painting. While planning may prevent plenty of minor mistakes from happening, planning too much may distract from the spontaneous creative stream of conciousness.
Here are a few tips on how to plan so that you are focusing on the bigger picture of the painting such as the overall story of the painting being told and what direction do you want to try to go in so your attention to irrelevant details isn't being spreadout too thin.
1) Decide on a Subject. Collect references & resources to keep around you so you have somthing to look at for more inspiration. If the subject is more vague, try doing several sketches to test out ideas and reach into what your inner artist is saying. Experiement with colors. Try different ideas, and learn from every brush stroke. Let every piece of work and interpretation be a reflection of yourself.
Through this, you'll find if you're the more "planning" or "go with the flow" type. The more of this you figure out about yourself, the more you can use it as a strength to make your painting all the more unique
3) Pay Attention to Size. ith these shapes, you also have to make sure that it all makes sense with the overall size of the painting. How big is the canvas you are using, and how much space do you want certain elements of your drawing to take up? Each brush size creates a different texture and shape, and each thing you paint will be different that what you painted next to it.
4) Colors. Use colors that speak the attitude of your painting. Instead of just using every color you can get your hands on, trying using a limited number of colors and different hues with each one to create a unity amongst the art. Each color technique will be different with each medium. An example of painting mediums are:
Paying much attention to the subject and the overall mood you want to create will help in the process of making sure you choose the perfect color every detail of your work.