If your question is about how to cut lengthwise, the answer is that you do it the same way you would if you were cutting across the fabric. you just have to do it more carefully.
you could try to fold it several times until you have a stack that will fit under your ruler. this is a pretty risky method since you'll most like end up with those lovely hills and valleys in your strips.
if i have to cut border strips down the length of the fabric, i sacrifice a bit of fabric and tear them down the length. i usually tear my strips at least two inches wider than i want them to end up. then, i start at one end and cut carefully along the ruler, moving it up a bit at a time, doing my best to keep the whole thing going in a straight line.
if you do it this way, make sure to leave at least one inch extra on each side of your border print. that way, you'll be able to cut it back to the print plus your seam allowance on either side.
may the force be with you and your ruler. :wink: