Lots of ways to do them. Here are a couple.
To engrave my laptop, I just scored a rectangle the same dimensions as the laptop, and placed the laptop aligned with the score marks. (This image was made later which is why you can see the burn marks, where I bled the image off of the laptop edge). Everything was aligned in the file, so it didn’t matter where the camera or overlay said it was.
I use jigs like this every single day to make puzzles. It’s a 12x20” piece of chipboard. The bottom edge is aligned to the zero mark of a pair of crumbtray rulers I designed (available in the free files section). I cut the puzzle outline out of the chipboard, remove it, glue the puzzle image and place it back in the cutout. I can take the jig out and work on other projects while the glue sets up, etc and just have to reference it back to the zero mark where I had it before.