Needing tips for arm overlays!

So I’m really hoping to use arm overlays in my up-coming story to add more detail. I’ve tried to do research on how to use them; before I completely mess up my script when writing, but haven’t found any tips or help with that specific topic. I was wondering if you needed to somehow remove the characters original arms and then place the overlays/overlay in, and then somehow get the arms back? Asking from images I’ve seen where it looks like they took out the characters original arms and then placed in an overlay. (Any help if appreciated, & if possible to show examples of script it would be very helpful!)