Where to begin, when painting abstractly? For me that's often the biggest challenge. When learning about the process in school, we studied Kandinsky's writing "On the Spiritual in Art", in which he recommends taking forms that occur in nature as a starting point. Really, though, anything can be a starting point. To prove it, I'll talk more about the process behind this painting:
It began with this photo, from some tourist literature for the Trentino region:
This is "speck", a kind of cured meat like bacon, I'm told. It's an interesting choice for a reference, because I'm vegetarian and the thought of eating this stuff makes my stomach turn...but the colors! The colors in the photo are beautiful.
As a reference for the underpainting, I turned this photo
upside down (this is very important, so that you'll see only the masses of color, and not a landscape or still life), and used that as a starting point. I massed in the colors quickly, but the result still reminded me too much of a landscape with a slab of bacon. Yuck. So, the next day (it's helpful to wait a bit so you'll see it objectively) I turned the painting on its side, and made a lot of big changes. I did not look at the photo any more after the initial upside-down session, because once the painting is started, the reference has done its job and is no longer useful. The painting will evolve on its own now. In the above finished painting, the red shapes are about all that's left of the original imagery--but they are the key to the painting. So, use anything as a starting point, and be willing to let it meander and evolve in whatever way will strengthen it.
Here's another reference (again, a tourist info photo, and again, turned upside down for massing in the shapes of color), and the underpainting for "Piccoli Frutti", posted last week.
This time, color was laid in with brush and palette knife, and the next day I worked back into it with a brush, enhancing the color and pushing and pulling the imagery. This is only semi-abstract, as it turned out. The shapes of the fruit were too seductive to leave out, but I didn't want to make them literal (see below, posted June 11).
Other ways to start an abstract include turning one of your own paintings upside down or sideways, and going from there. It can be hard to break the mindset that you had when you first painted it though, so a sketch or photo might be a better way to go.