Add canvas layer over the existing one to draw on it.

[TBoxhead]

What would be the method to add a canvas element on top of the existing one so that i can draw shapes on it? Do i use the custom render? because the documentation is not that clear to me. I want to be able to draw rectangles and draw arrows and connect rectangle annotations or even highlights and i want a blank canvas which i can work with without messing up the one with the pdf