Mounting them is probably easier than finding the mounts or a projector to put them in
That is, projectors that can take 6x6 slides were never common, and are less so now, and mounts may not be available either.
There are (were) two kinds of mounts the "Gepe" style plastic mounts and cardboard that had heat seal adhesive pre applied. For the Gepe you either slide the film through a slot of the assembled mount, or there were also two-part mounts where you position the slide then snap the other half in place.
For the cardboard mounts you would place the slide, fold the mount over then melt the adhesive with a clothes iron or a dry mount tacking iron.
Either way, as you understand, it's a matter of carefully trimming the film to fit, how easy that is depends on your skill. Dedicated cutters exist (existed).
Sad to say but the best way to deal with them is probably by scanning, but they do look nice projected onto a proper screen.
A possible more viable alternative is to use the "Super slide" format which is more or less 4.5 cm square. These fit in the same projectors that can project 35 mm slides. They take more trimming, and you may want to use a mask in your camea for composing, but you have more choices for the projectors. Not sure if the mounts are available though.