How to Mount Slides
The film will come back from the lab uncut (because you remembered to request this) and in a plastic sleeve. It’s important to keep this sleeve on the film.
Take a look at the photos using a light table and a loupe, both of which should be available for use at wherever you had the film processed. Start at the end of the roll, and mark every left image of each stereo pair with one of the small stickers (the sticker should be placed on the plastic sleeve, not on the film itself). When you later mount these stereo pairs, this sticker will indicate which image will go on the right side of the stereo slide mount. (For the 3D effect to work, you need to switch the left and right images before mounting.)
You can also take this opportunity to mark the images that you think will most be worth mounting, using the fine, soft-tipped marker on the small stickers. I use a simple system of one dot if it is worth mounting at all, two if it seems promising, and three if it seems like a keeper. Since mounting is costly and time-consuming, weeding out the uninteresting images now will save you unnecessary hassle later.
Next, cut the film, still leaving it in the sleeve. Dull scissors will ruin your images, so use sharp ones! I usually do this at the light table at the lab, but if you do this at home you might consider getting a lightbox. Because the gaps between frames can be narrow and hard to see, I recommend using the perfs as a guide. Look at where you begin your cut relative the the perf that is closest to it, and aim for the same place relative to the perf across from it on the other side of the film. If you end up clipping a bit of your photo, it’s not the end of the world as the mounts do crop the image somewhat. But after you’ve gone through a roll or two, this will only happen if you’re being careless. You should end up with 58 individual frames (29 stereo pairs), each still in it’s own segment of plastic sleeve.
(Some people recommend using film cutters for extra-precise cuts, but I think it’s unnecessary. More importantly, it adds to the hassle because you have to remove the protective sleeve to even get the film in the cutter.)
As you cut the film, put each stereo pair in it’s own section in a sorting tray. I use this, as it has a latched lid that keeps the everything separate when I carry it around, and it has more than the 29 sections you would need to contain a full roll’s worth of stereo pairs. Put the unmarked frame of each stereo pair on the bottom, and the sticker-marked one on top. This way you can see how you’ve labeled your pairs at a glance.
Now it’s time to actually mount the slides. Since you will be handling the film directly, put on cotton gloves. Pry open the two halves of the RBT mount, and place the supplied pinbars into the grooves above each of the windows in the bottom (black) half of the mount. Remove the sleeve from the unlabeled image of the stereo pair you are mounting, and place it on the pinbar over the left window. Then remove the sticker-marked sleeve from the image you’d labeled, and place it on the pinbar over the right window. Take the top (white) half of the mount, and place it over the bottom half to which the pinbars and film is attached. There is a faint groove along one side of this top half to indicate where the bottom of the image is, so make sure this matches with the orientation of your photo. Align the small rods/holes and press the two halves together to snap the mount shut.
If you’re so inclined, you can label the slide mounts with larger stickers, placed between the images. You should also save the plastic sleeve segments that you removed from the film before mounting, so you can put them back on to your stereo pairs if you ever unmount them.
Place the stereo slide in your viewer, with the white half towards you, and the groove on the bottom. Hold the button to illuminate, and adjust your interocular distance and focus as necessary.