Unfortunately it's one of those practices where you don't get good forever eventually, it's a lot about keeping your hands excercised so you would spend a lot of time re-doing those sheets if you wanted perfect lines at all times.
Aside from ruler, which is fine, it's much better to work around your hand movement - position your other arm around as an anchor, hold the paper in a way your wrist won't sway, flip it around to the one direction you can do your best straight line. It's not a contest and there's no harm in using things to be more efficient and comfortable.