I began this drawing using the broad nib of a double-tipped calligraphy marker. I worked with a variety of shades of blue, creating a free-form image with a central focus. Then I added some aqua and pale greens.
Next I switched to the narrower point of the calligraphy pens to draw closely-packed parallel lines in order to create texture and more densitiy in some of the shapes. I also used a fine point black pen to outline some of the shapes in the center of the image, drawing the eye toward that center.
My final step was to fill in some of the areas with a pale green brush pen.