How do I delete hand drawn doodles without deleting whole note?

Deleting a sketch should be easy. While you can use the eraser tool to selectively delete drawn elements, this won't erase the full sketch/doodle.

To remove a sketch from your note, just double-tap with your finger to select all of the objects in the sketch. Next touch-and-hold with your finger (i.e., a long-touch) to expose the Action bar/menu; select Delete.

That's it. You're done.

Select the eraser tipped pen from the gallery that appears at the bottom of the note.