A good exercise for this is to place two dots on the page then with your pen not touching the page, go through the motion of drawing a line connecting the points. Do that motion a few times then commit & draw the line. Go over it a few times trying to be consistent. focus on starting & stopping right on the points. Try to do it with only the pen touching the paper & focus on the feeling of your arm & shoulder doing the work. After, check your work with a ruler & a red pen to see how you did. Another exercise is to use a ruler to draw some long lines then go over them free hand & try to match them.

here is a better scan of this project.