Initial layer: The light tones, where you block in the main colors. Because we are working from light to dark, colors need to have been thinned with water first. No need to worry about shadows this early in a painting.
In the forest painting, I did start adding shadows as I wanted to set a moody atmosphere.
In a sunny landscape for example, I would not worry dark tones at this point.
Start with creamy mixes of paint (neither watery not thick). This ensures that colors flow easily on my initial layer and that they look light since they have a lot of water in them, and it's also great to make sure they stay vibrant enough that we don't need to add many layers on top.
Second layer: The mid tones, where you start building contrast. Start working with the same mixes but add more pigment to make them a bit thicker. You can also start leveraging color mixing to mix darker versions of some of your colors and start placing shadows on the painting.
See how I did that in the forest painting, by adding dark tones all around and adding dark trees in the background.
Be careful not to cover up all of the first layer so light tones still show through in places.
The light tones stay most visible in the center in my painting, for example. You decide.
I wanted to create a certain mood in my painting so I tweaked my reference photo by adding more darks and increasing the overall contrast.
Third layer: Details and dark shadows, highlights. Working on specific areas.
In this painting's third layer, I added dark branches. I also started adding foliage to add a bit of color and brighten up the piece.