Squaring off is what I meant by trimming. No you dont have to starch but if you have a thinner fabric it sure helps alot. Remember the ruler is alot like a measuring cup, you have to look directly down on it to read it properly. I also use an Olipfa with the lip which helps.
Keeping the blade perpendicular sounds so easy until you try to do it. You might be turning your wrist as you pull up toward the end.
Practice in front of somebody who teaches or works at your LQS. They will be able to correct any bad mechanics in a heartbeat.