Before I play I make a custom game, put in three Lucio hard bots, three Ana hard bots, turn on headshots only. Then for 5 minutes I focus on scoping in, keeping my crosshair over a head for a few seconds, repeat, then I start taking flick shots for 5 minutes. After that do all the same things while practicing erratic movements. The same drills probably work for Ana
(though you won't be able to kill them unless you do the drills as widow) additionally something that holds a lot of people back is having a sens that is way too high and being unwilling to do the one step back to get the two steps forward of lowering their sens and getting used to it.
It is like wax on wax off, it is simple and repetitive but it ingrains good habits and muscle memory.
Google just gave me this:
https://www.youtube.com/watch?v=erBcxzynAI0