I use an eyedropper to make long straight lines. I mix the color I want to use, fill the eyedropper, place it where I what a straight line, and — because I paint with my paper vertical — I let gravity do the rest.
I paint very large and tack my paper to the wall to paint the bridges for which I’m known. This eyedropper technique is perfect for creating vertical cables. If I want other lines in differing directions, I turn the paper as needed.