1. The card mount is placed on a light box with the rough inside surface of the cardboard facing you. The rounded apertures can be either at the top or the bottom as shown in Fig 1a or 1b, depending on whether you prefer the tape that seals the mount to be across the bottom or top of the mount.
The film chips are placed on to the card with the image back-to-front, (emulsion side towards you), but right side up.
It is important place the film chips over the apertures with the rounded corners, as these smaller apertures are the ones that form the stereo window. Hence, the film chips should be taped down on this side of the mount. (The apertures with the square-cut corners are slighty bigger, and taping the film chips to this side of the mount will introduces less precision into the setting of the stereo window.)
2. The position of the film chips relative to the apertures determines the correct setting of the stereo window. There are different methods, and different opinions on the setting of the stereo window. The method used by the author is presented here, and for further information the reader is referred to "The World of 3D" by Jac Ferewada. If you have an established method for determining the setting of the stereo window, then stick with that.
Ideally, it is best to obtain or make a jig to assist in positioning the mounts. Such jigs will not be discussed here. However the author has found it very useful to provide a "reference ledge" to set the 3mm dimension shown, and a clear plastic hinged flap to go over the chips to hold them down while positioning them horizontally.
Adjust the height of each film chip so that the two are aligned at the same height relative to the film apertures, without twist. Nomally this results in approximately 3mm between the edge of the mount and the edge of the film chip.
The near point separation should be adjusted so that it is equal to or greater than the aperture separation. (The near point separation is the distance between the left and right image of the closest object in the scene.) If the near point separation is too small, objects appear in front of the window, so the film chips should be pushed apart.
Ideally the far point separation should not exceed 63.4mm. So this means that the film chips cannot be pushed apart too far. For hand viewing, it can be acceptable to exceed the 63.4mm far point distance, but for projection it is more important to try and satisfy this constraint.
If the slide cannot be adjusted to satisfy these rules, then the depth-range in the photo is too large - the photo should have been taken with less depth, or sometimes special mounting techniques can rectify the problem (see "double-depth" method by Ferwada).
At first glance these rules might make you think you will need to measure every stereo slide you mount with a ruler. However these rules can be obeyed by learning to look for the right things when mounting. In short - the near point can be adjusted by ensuring that just a little more subject material is visible to the right side of the left near-point-image compared to the right side of the right near-point-image (and similarly for the left side of the right near-point-image compared to the left side of the left near-point-image).
3. The film chips are taped into position. To allow for any possible expansion of the film chip from heat in the projector, it is best to tape only along one edge of the film chip. This will minimise the chances of the film "popping" in the projector, and causing focussing difficulties.
It is good to use a high quality adhesive tape at this stage, such as Scotch #850, approximatley 8mm or 10mm wide. The adhesive in this tape will not "bleed" from the edges over time.
4. The finished mount as seen from the front is shown in Figure 4 - the way it is inserted into a hand stereo viewer. The smaller apertures with rounded corners face the front. A "spot" marker can be added if desired, in the standard lower left hand corner, to indicate slide orientation as placed in the hand viewer. The mount is sealed with adhesive tape across the open end and trimmed as shown - an inexpesive type such as Scotch #688 will be OK.