To mount a photographic film slitmask onto a holder you will need: a light table; scissors; narrow black tape (for blocking out the mask border); and a jeweler's screwdriver. All except for the black tape can be provided by the day crew. The tape should be bought in town; good examples are ``Letraline 476 3/16-inch black matt 3730'' or ``Letraline 96 1/4-inch black flex 2426''. Call around for a graphic design store to find such tape. You may also use the black photographic tape which is kept in the wooden desk just outside the control room, on the observing floor.
Using scissors, trim the slitmask film to keep ~1/4 inch outside the rectangular mask border. Also, diagonally cut off the four corners of the film.
Take a mask holder and remove the cover (there should be four screws holding it in position). There are three notches cut into the outer ring of the holder. One notch is closest to a short side of the rectangular mask hole; place this notch facing towards your left. Turn your slitmask upside down so that the label reads backwards and lies in the upper right corner, and place on top of the mask holder. Place the cover on top, and lightly fix in place with the screws, leaving it loose enough so that you can still move the film around.
Place the mask holder on the light table. Slide the film around until the upper/lower mask border lines up with the upper/lower edge of the rectangular cutout of the mask holder. This is a crucial step to do carefully, as it sets the rotation angle of the slits in the mask. When you are satisfied, tighten the four screws to fasten the film in place.
Use the black tape to cover the four clear borders of the film and the label. Also cover any imperfections in the black portion of the emulsion.