I begin by paying attention to the thoughts I have when I am not writing. These appear at any time as I am sure you know. It is usually when I am having a shower or ironing shirts or sitting on buses. I have got in the habit of writing these on slips of paper or in a notebook. Later on, I add these notes to a folder. Once this folder grows fat with thoughts, I go through them, pick out the best, and discard the rest.
I am looking for the still-burning embers of ideas among the ashes of my thoughts. You know when something is red hot. Ouch! These are the ideas that reignite the fire of my imagination. I look for resonance. I look for interesting juxtapositions.
The idea is the seed that germinates on the page. The point of my process is to take wildflowers of ideas and cultivate them so that they may grow and flourish. Light shines on them, and trendrils grow and stretch into our world, and flowers bloom. And so with writing, these blossoms may become immortal.