That answer will vary from artist to artist. But the answers I hear most often are "I have to be inspired" or "i go through my photographs and find one that looks good to me" Or maybe even, "I start with a value sketch and a color map."
All good answers.
But one thing that often happens to me when I'm about to start a painting or a series of paintings is that I have moments (even days) of apprehension. "What am I going to paint!" "What will inspire me?" and I usually run around thinking it through and wasting valuable painting time.
But what if we use that time to experiment - with our paints or colors or brush strokes? What about wet on wet, throwing paint, ink pen, watercolor pencil?
We usually have small pieces of paper that are laying around - too small to be a good painting but also too big to throw away. So what if we use them up learning things about our media.
This is where trash bin painting comes into play.
Use those end pieces of paper to experiment with triads. Pick three colors only. Choose one as the dominate color. You can create many more colors by mixing, either on the palette or on the paper, these three. Don't have a preconceived idea of what you're going to be creating. Just play!
Then do another: trade out one of the three and do it again.
Try to be as loose as possible on this. Create only with the idea that if it isn't any good, it can always be tossed into the trash bin.
But i usually save these - just because looking back through them, I can see something that worked or didn't work and with time, it becomes obvious why.
And the best part, it loosens us up and sparks our creativity. No judgement - no pressure.