I will show here a little of the preparation and composition studies I do before I commit to canvas and paint.
(a: base) I begin with a photo.

This photo has been squared, because I decided to paint this work on a 12" x 12" canvas. This what I call my "base" photo. The first step is to scan it and open it in Adobe Photoshop as an RGB file and make it a size I can run out of the letter-size color printer (but I am not ready to use the printer yet.)
(1) I use Photoshop tools to manipulate the colors and the effects. In Photoshop there is a group of filters called "Filter Gallery." I open the photo as an RGB file, the go to Filter Gallery and select "Paint Daubs." That filter alters the photo to appear as if it had been painted with a pallette knife. I also use the color adjustments in Photoshop to brighten and contrast the image. Sometimes I adjust the hue to add purple or to cut down reds, cyans (blue) and yellows. I reached the point where the image suited me. Except for the colors and sky - which would come next. Here is the image at step 1.

(2) I selected the background sky using the magic wand selection tool, set at 18 pixels. That selected only the area I wished to modify. Then I opened another photo with a brilliant blue sky filled with scudding white cumulus clouds, changed the size to match the size of my poplars sky area, and finally copied the sky. Next I went back to the poplars "paint daubed" photo and used "paste into" to drop the copied sky into the selected area. This is what I got...

(3) Next, I stretched the new sky until I got it to fit the image and look just like I wished it to look. Then I flattened the image so everything was now in a single layer. Once that was done, I returned to the Filter Gallery and Paint Daubed it again, using a sharper "sharpness" setting so that it showed the edges of the colors. Here it is, just the way I wanted it to look before it went to my printer.

Now I take the printed image, a square about 700 pixels by 700, on a letter-size piece of paper, up to my canvas. I sketch the main outlines of the image in light pencil. Then I begin to paint, using the new and modified image as my model. I don't always paint it exactly as the printed image appears. Sometimes I strengthen or even change colors. It all depends on the muse and sometimes even fortuitous errors. And how I feel, of course.
Demo over.