Summary

A 90 degree bracket with two screw holes on each arm.

Why yes, you can just buy these but where is the fun in that?

Print settings... well its your shelf, how strong do you want it? Use your noggin and remember all shelves are hung at your own risk! (This is just a general life lesson, 3D printed or otherwise!)

EDIT: Photo of finished shelf added, if you want to 3D print your own shelf to go on these, try this:

https://www.thingiverse.com/thing:4341778