Sometimes when I sit down to do a painting I'll have been thinking about it for some time in my head about what I want the subject of the painting to be, what type of setting and lighting etc. However for this particular painting all I knew was I wanted to do a picture of a Mermaid. I start in my sketchbook working out a few pose ideas, VERY loose. At this point I don't even know what type of setting I'm doing, I'm just concentrating on the figure. I wasn't liking where this was going, so I took it even more basic and did some quick compositional thumbnails, from which I was able to select a general "mood" for the subject.
Step 2: Refining the Concept:
Once I understand the general flow of the painting, I start to really concentrate on composition and the figure. I knew I wanted her to be situated on the floor, raising herself up on her arms, so here I work out the general look I'm going for. I decided on the sketch on the right. From a composition standpoint I knew I wanted something to the figures right back to complete the flow of the painting and bring the viewers eye back to the main subject, but at this point I was still unsure of what that was going to be.