What are some good rules of thumb when installing windowsills / windowstools?

Moving into a new apartment that has drywall returns already installed around the windows, but does not have any window framing or sills (I understand they’re called stools when on the interior).

I’d like to add in sills / framing, but I am not sure if I need to remove or reduce the drywall returns first, or if I can add framing and a sill on top of what is already there. There is concern that sills that are too thick will impede the ability to open the windows themselves (double-hung, grab bar at the bottom to open / close).

What is good to know when approaching this kind of project?