For a while now I've been thinking about "simplification". As a plein air painter, you have a limited time to get as scene. In theory you should be able to tackle anything but some painters will limit the scope of the scene to limit the confusion. If you were in a farmer's market for example, you could choose to paint the whole scene with crowds and all or just one stand or even one single orange. Simplification will be needed regardless of the scope. -and the orange is just as difficult a the whole market.
We can define simplification by what it is NOT. It is not carelessness or hurrying. As a matter of fact, to be fast and simple you will need to spend a lot more time preparing your attack than throwing paint willy-nilly.
The first thing I always do is lay the horizon line, never in the middle, but thinking whether I want to emphasize the sky or the land below. It amazes me how many people ignore basic perspective.
I lay down a precise and accurate position "map", I pick some key points in the scene and measure and measure again. Is the span of the dark arch a wide as the receding wall, is the distance between the tree to the right and the building twice or three times that span. Amateur painters and some experienced ones often make their subject of interest a lot larger than it is or try to squeeze things in at the last minute when they should have taken the time to measure. This results is awkward paintings and wasted time.
Then, I figure out where are my darker darks and my lighter lights, my softer edges and my harder edges. I don't care what things actually are, I don't see leaves or bricks, all I see is soft edges and had edges. UNTIL you can speak out loud where this things are, don't even bother.
And then I lay down accurate averages of the colors I see, not the local colors but the actual colors.
Only the last hour is devoted to quickly and precisely fill in the gaps and details. All the guess work is gone or very limited. By the way, more and more I use my grid view-finder, my red acetate gel and my color charts. No shame there, it is not cheating. And every time I use these devices, a better painting session is almost guaranteed.
So simplification, the process of reducing infinite detail to the main shapes is just that, KNOWING what the main shapes, colors and forms are so you can achieve freedom from details while you paint.
NOTE: A gridded view finder is a piece of plastic you hold up to find the best composition. Not the most sophisticated tool ever but some of them have a superimposed printed grid that allows to place the different elements according to a grid you can draw in your canvas mimicking the one you see.