There are more than three steps to heaven
I don't have a clear artistic vision yet: I am still learning. Every painting is an experiment from which I learn. However, in my research on the subject of simplifying paintings I have learnt one or two things that I can pass on to you.
My painting of the steps at the Abbey in Saint Hilaire was created in three simple* stages so I thought I would use it to illustrate this article. I wanted the painting to be one of light and shade so in the first stage I used a light tone in a cool colour to help me see the tonal arrangement. I did not use masking fluid to retain the light tones so it was essential that they were easy to see when I came to adding the shadow, as that needed to be in one fluid wash. In the second stage I established the warmth over the whole picture and in the third stage I painted in the shadows.
Reducing detail is the obvious answer to simplifying paintings but it can be challenging deciding what to leave out.
To help you decide try the following:-
Identify your focal point - it's okay to have detail at the focal point!
Then reduce the image to major shapes. Make a tonal sketch and work from it when you come to paint.
Use as few tones as possible. Try only painting the shadows or silhouetting areas. Link areas of the same tonal value together. For example, try linking groups of figures, trees, objects in a still life, or flowers in an arrangement. A variety of colours can be used within the shapes for interest.
When you paint use a big brush (and a small piece of paper, if necessary!) this will force you to stop painting detail.
Work within a time limit. Again this will force you to make a decision of what absolutely has to be included in the painting. Once you've painted that, ask yourself whether it needs any more. Having a plan in place before painting is time well spent when working within a time limit.
Challenges and other articles that deal with this topic are:-
Tone Challenge - painting with one colour
The Thumbnail Challenge
Planning a Watercolour Painting
*Don't be mistaken; "simple" does not necessarily mean easy!