I mounted a few 645 slides a couple of months ago as I couldn't find a lab to do it for me.
Perfectly straightforward to do with a sharp pair of scissors, decent light and a pair of gloves to help avoid fingermarks (I used nitriles as i do in the darkroom).
I used Gepe mounts, which are plastic and come in two parts. They snap together - no glue. They can be taken apart again, but I found it a faff. Not sure why one would want to do so anyway (I only did so on one or two because I had to rejig the placement).
If I were doing a lot regularly, perhaps a mounting press would be convenient , but it's not absolutely necessary.