Inserting and removing the masks from the instrument is a difficult process, so it is required that you be instructed by a member of the day crew or an experienced observer.
As you enter the prime focus cage of the 200-inch telescope, move around counter-clockwise to provide easy access to the blue box (the COSMIC local motor control console). Assuming the zipper on the COSMIC dark shroud is in its usual place, you also then will have easy access to a velcro-closed opening in the shroud at about waist level. Open this to get access to the aperture wheel. The first time you try this you might want to unzip the zipper on the COSMIC dark shroud, so that you can see the entire instrument more easily. (Sometimes the setup crew leaves this shroud in a position rotated from its correction orientation, so you might have to remove the shroud anyway.)
Use the blue box to move the desired aperture wheel position to the install location. First push ``AP'' to select the aperture motor, then push ``INC'' or ``DEC'' to move the wheel. The position displayed by the blue box is the position of the aperture wheel that is in the optical path; the position in the installation location at that time is 2 positions less (modulo 6).
Reach over the blue motor housing mounted just above the aperture wheel on COSMIC. Hold the holder so the ``locating notch'' is pointing away from the blue motor housing. Insert the holder and mask so that the 3 notches all clear, then press down, and turn clockwise approximately 1/4 inch, after which you will hit a stop. The stop is defined by knife blades hitting the beveled edge of the each of holder notches. For peace of mind, tape the holder to the aperture wheel once the holder is installed.
Repeat this procedure using the blue box to move to each desired position on the aperture wheel for each mask you plan to use.
Be sure to close the velcro flap on the shroud prior to leaving the prime focus cage.
Once you are back in the control room, cycle through several different positions on the aperture wheel to reset it properly.