To draw straight lines on the iPad in apps that don’t have a ruler tool, I use whatever straight edge I have laying around: plastic ruler, book cover, piece of cardboard, notepad, etc.
In the Paper app specifically I have journal page templates of “frames” that I duplicate1 anytime I need them. Saves me the step of having to redraw the same straight lines over and over again.
With the addition of the Think Kit tools in Paper drawing straight lines and shapes is even easier (no rulers needed). Select the Diagram pen and watch as your scribbles autocorrect into perfectly formed shapes.
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. Then tap that.↩