There is a drawing practice, were squares are used for practicing certain patters, like horizontal/vertical lines, diagonals, curves and hatching (overlaying patterns to define shadows).

I created a OpenSCAD script that can be modified to your fitting, from the size of the sheet, to size of the square, margins and padding. I also added an STL as an example, that fitted my sketchbook and also my printer (a bit smaller than my sketchbook). 

I hope you find this helpful for your drawing practices.