How accessing the canvas of a atdiagram and drawing on it?

My mission is to produce "hand wriing" by dragging the mouse over the diagram while the left button is still pressed down, and save the result. I need this for a teaching purpose using SmartBoards.
Please if I may ask you to dirct me with few words to select the best approche to begin with.
I know I should access the canvas of the AtDiagram or one of it's blocks but I dont know how?
Please Help
an Thank you in advance

Maybe this kind of link can give some insights?