To draw straight lines on the iPad I use whatever straight edge I have laying around: ruler, book, piece of cardboard, notepad, etc. I have a few Paper by FiftyThree journal pages of “frames” that I duplicate1 anytime I need them. Saves me the step of having to redraw the same thing over and over again.
Think Kit update
With the addition of the Think Kit tools in Paper drawing straight lines and shapes is even easier (no rulers needed). Simply select the Diagram pen and watch as your scribbles autocorrect them self.
To copy a page pinch the screen to zoom out and then press and hold on the circle plus button until a duplicate button appears. The tap that. ↩