First, the gathering of raw materials-both the materials of your immediate problem and the materials which come from a constant enrichment of your store of general knowledge. Second, the working over of these materials in your mind. Third, the incubating stage, where you let something beside the conscious mind do the work of synthesis. Fourth, the actual birth of the Idea-the “Eureka! I have it!” stage. And fifth, the final shaping and development of the idea to practical usefulness.
Source : A Technique for Producing Ideas