Laying the groundwork for a painting requires some patience but I am usually very happy with my work when I take the time to do some planning.
I start by selecting an interesting composition. Usually when I am in the field I carry my phone and use the camera feature to take several photos from a few different points of view. When I have something interesting, I set to work with the paint.
I usually tone the canvas first (laying in a thin layer of paint and turpentine) to kill the whiteness of the canvas. I may sketch a few marks with a pencil on the canvas to get the large shapes, and then I use a single paint color to build a value sketch of the subject. Values sketches are like the framing of a house--very important to holding up everything else.
Next I begin to add other colors, careful to observe that the values (lightness or darkness) matches what I had in my underpainting. I lay in large blocks first with a large brush, trying to be loose and quick. The aim is to capture the light condition.
As the painting progresses I stand back to observe. What looks right and what needs fixing? Are the proportions right and is the composition still interesting. Now is the time to move things around.
Towards the end I move towards smaller details with smaller brushes. It's tempting to get fussy here and forget to let the brushstrokes show.
I usually spend 2-3 hours at a time working on a painting. Sometimes I am able to finish, and other times I have to let it dry and return to it another time. The paint can build up and become muddy if worked too much. In any case, sometimes I don't know if I am really done until I get away from the painting for a few days and then look at it again with fresh eyes.