A little while ago, I played around with TikZ to make printable paper. As it turns out, this is absolutely trivial, and with just a few lines you can generate pretty much anything you can imagine. Here are the examples that I generated:

A4 paper

Blank Paper
Ruled Paper
Graph Paper
Underlay Paper

Letter size paper

Cornell Paper
Ruled Paper
Graph Paper
Underlay Paper

You can adapt any of these examples to your needs easily, as well as create more complicated layouts. Anything you can find on http://www.printablepaper.net/, for sure. To compile, run the tex file through pdflatex twice.

Goodnotes

Moving beyond "printable" paper to "virtual" paper, the Goodnotes app allows to define custom templates. The following template was created so that for a fully zoomed out letter-sized page on a 13 inch iPad Pro screen, the grid is exactly 5x5 mm on screen. This way, I can use a physical ruler on top of the iPad screen.