Those are a good first try! Pay close attention to marking and staying on the waste side of your lines and you'll have it in no time. Are you using a knife to mark those or a pencil? If you are using a pencil, that is probably part of your error. Even using a 0.5 pencil is not nearly as accurate as a knife. I fought with fit when I used a pencil for marking and couldn't figure out why - switched to a knife and most of my problems went away.
"Good Behavior is the last refuge of mediocrity" -- Henry S. Haskins