I also have to confess that this is my least favorite step of the whole quilting process.
I start thinking about how I am going to quilt a piece as soon as I start sewing the fabrics together. Some decisions are obvious and easy, others are more of a problem.
I also do not try to mark my fabrics as it will just be a line that I will not be able to follow and I do not want that pressure of concentrating on sewing an exact line. I much prefer picking a general area to cover or using the edge of my embroidery foot as a guide to make further parallel lines as shown in the photos here.
With the subject of birds in flight I wanted to use flowing lines. And since the birds themselves are fairly small, I wanted to leave them unquilted so they would "pop" from the surface of the finished quilt. So I have decided on choosing various areas of the background to heavily (for me) quilt groupings of parallel lines.