Sounds simple doesn't it? Draw a straight line. Go ahead, give it a try. Take your pencil and draw a straight line. Hint: drawing a line side to side is easier then up and down. Not easy is it? Now, try a ruler. In technical theater we use rulers all the time. You don't have one? How about the edge of an index card? Maybe the edge of that math book you want to throw across the room. You have a lot of straight edges you could use in place of a ruler.
Several tools are available to use to draw our lines.