If you want the overlay already in the scene you add:
ex)
INT. BACKGROUND NAME - DAY with OVERLAYNAME
If you want the overlay in a specific location (spot directing) just do:
ex)
INT. BACKGROUND NAME - DAY with OVERLAYNAME to % x y in zone # at layer #
~~
If you want your overlay in the middle of a scene put:
ex)
@overlay OVERLAYNAME create
and add the opacity
@overlay OVERLAYNAME opacity # in T
if this doesn’t make sense check this website for help: