I used the Magnetic lasso tool. On the tool's option bar, I set contrast to 3%, width 1px, frequency 57. When using this tool, you can lay down an anchor point at any time by left clicking, going further along, left clicking again, etc. But, you can also go back by hitting the delete key on the keyboard. It is helpful to work under magnification - use the scroll wheel of the mouse or the zoom tool (Z).