Overlay Finding:
Well, I usually find overlays for my story by using a website called kisspng. You can check it out if you’d like! Here is the link! You can also find overlays from plenty of different people on instagram and the forums. There is always the internet as well! Just look up an image and remove the background with a background removing site!
How to place an overlay:
It’s kind of like putting weather effects in a scene. I’ll show an example
INT/EXT. BACKGROUND - DAY/NIGHT with OVERLAYNAME to COORDINATES
Here is an example from a real episode script:

More than one overlay:
It’s quite simple really. To have more than one overlay, you just have to put with OVERLAYNAME next to the overlay you already placed. Like this:
INT/EXT. BACKGROUND - DAY/NIGHT with OVERLAYNAME to COORDINATES with OVERLAYNAME to COORDINATES
You can type that command in as many times as needed I believe.
Do I Speak French?
Oui, pourquoi en effet je fais 
I hope this helped you out in some way! If you need need more help, there are lots of different threads on this platform that might help you out!