Look at the subject (here a landscape). Study it. Forget names of things (e.g. sky, tree, cloud) and look for shape, color, design, and value.
Squint, squint and squint again. Squinting helps eliminate detail and reduce color so you can see the big shapes and movement in the image.
See it already painted in your mind. See the forms of your subject in two dimensions.
Do not rush this step. Three-quarters of the painting is done at this stage.
