For my instax wide I keep the dark slide that is ejected for the first picture for this scenario.

Unload film packs in complete darkness.

Open the film door, and remove the pack.

Carefully feel (with just washed and fully dried hands) for the first sheet of film.

Slide the dark slide in in front of this first piece of film.

Once the slide is fully seated, lights can come back on.

The other trick is to put a post it on the back of the pack to remind you how many shots are left in the pack.

The next time you load a part full pack in an instax, it spits out the darkslide, and presumes there are 10 shots ( a full pack ) after the dark slide.

The post it, transferred to the back of the camera, reminds you this is not the case.