Brainstorming:

After coming up with my idea I closed my eyes and took some time to envision it in my mind. From here I decided on a brainstorming session where I made a very crude sketch and scribbled down all the characteristics I wanted to feature in the painting. Aside from the obvious features and colour choices, I got the idea to make hell appear jagged and sharp in contrast to heaven's smooth and rounded appearance. This is a traditional art technique known as "form language" where shapes are used to create moods and to represent certain characteristics.

I also decided to give the female wings and make her appear as an angel - widely recognised symbolism that would tell the viewer that he/she is looking at heaven and hell. I then decided her wings should be on fire, indicating that she can no longer return to heaven and that she has made the ultimate sacrifice for her lover. However, some people may view this as punishment for her defection - a statement about God's spite and intolerance. I felt that such touches worked well because they were open to interpretation and appealed to religious and non-religious people.

My original intention was for it to be viewed as a romantic gesture, but I realised it could also be interpreted as something much darker, that the male figure could in fact be a demon, deceiving the angel and luring her into hell. It would also raise questions about why her lover is in hell in the first place. With a painting that is so clearly about two extremes, I liked the idea of separating the optimistic viewer from the pessimistic viewer, the believer from the atheist and so on. I knew then that it was important to keep the symbolism subtle and not to spell anything out to the viewer. 
2/10
My idea had complexity and worked on many levels, but they didn't all come at once. Big ideas never do! Some of the meanings developed from my brainstorming session, while others were pure coincidence.

The key was to find my starting point and to build on a basic theme. One idea inspired another as I gained momentum and the concept started developing into something more detailed and deep.

The sketching process helped me to figure my layout and give me some idea where all my elements would fit. No need to work in any detail at this point because I knew things would be changing a lot throughout the course of the painting.

My next step was to start thinking about colours and tones, and a more structured layout.

This I would achieve via a series of thumbnails...