I've been thinking that at least for me, a painting can be broken down to several parts. The first, and most important, is the idea, or as they said at my college, the concept. The idea can come in a flash, or it can take a lot of thinking time.
The next part is finding or creating the image. I've been working on my Sylvia paintings lately, and I usually physically put her in the situations or places so I can photograph her to get my image. Sometimes I combine a photo of her in position to interact with an image I find online. That brings us to the next part, which is fine tuning the image on the computer.I use photoshop to get the images to work with scale or brightness. Sometimes I eliminate the background to put the attention on the figures.
The next part is getting the image on the canvas.I used to print out the image from the computer, graph it out, the draw a graph on the canvas, then hand draw the image block by block. Now I have a projection tool that I can use to draw the image. I just do the basic outlines, it insures the proportions will be right.
Now we are ready to get some paint on the canvas. I usually block in the colors, making sure they work together. Then I build up some layers of paint to make things more solid. I like to paint the background first and then move forward. All along the painting process is some tweaking with color and value, (brightness and darkness).
The last part is deciding when I am done. Sometimes I really just want it to be done, and I have to force myself to take a good look to make sure it all reads well, and I have succeeded in showing what I wanted it to be. I am not a painter that does super detailed work, but I admire those who are. I feel that paint is paint, it's crazy to try to convince someone otherwise. When I am done with a painting I like to have the next one ready to be worked on.