This sea painting demo was done without any preliminary sketch of the composition on the canvas, but don't assume that this was because I went straight from blank canvas to what you see photo 1. Before I put brush to canvas, I had been doing a lot of visualizing and planning in my head, following days spent observing and photographing the waves on a small stretch of coast, and painting some sea studies.
One of the first decisions was what shape / size canvas I would use. I knew I wanted to use a landscape format, because it fitted how I'd visualized my subject. I picked one about a third as wide again as it was tall (120x160 cm / 47x63") from those I had in my studio.
Once I'd selected a canvas, I then decided how I was going to position the wave on the canvas. My intention was to paint a close up or a small section of a breaking wave, with the breaking crest and foam of the wave dominating the scene. Next decision: would the wave be breaking to the left or right. Only then did I then put brush to canvas. (See also: Six Things To Decide Before Starting to Paint.)
The first step, to establish the composition of the painting by putting down the basic shapes, lights and darks, was done using titanium white and phtalo turquoise only. (I was using acrylics.) Notice how even at this early stage I'm not applying the paint haphazardly, but in directions relevant to what I'm painting. I'm doing this because I know I'm going to be painting with glazes, which means that lower layers in the painting will show through. While I don't know exactly how many layers or glazes I'll be using, by painting "in the direction of growth" right from the start, I don't have to worry about it.
Once I had the basic composition sorted, I switched to Prussian blue to add darks in the background, and then foreground. (Photo 2).