here's a video how to load a pack film into the polaroid 405 holder:
Once the black tab is pulled out, do not open the holder.
Put the holder on the back of the camera, close shutter,
open the dark slide, expose, pull back slide-
pull out the little white tab with the number 1 (a second tab with number 2 will appear),
pull out the bigger white cone-shaped tab with one constant movement .
For the mask: I'd recommend to use one. Only I've never seen one of these masks on ebay or elsewhere. Maybe better you cut it yourself:
Tape two long white paper stripes onto a wall, one vertical and the other horizontal.
Mark each centimeter or half inch and number it, big enough that you see it afterwards on the instant film. Mark only the areas where the image area ends roughly. Or take measuring tapes (eg.from IKEA).
Put the camera in front of the wall with a normal focal length or longer focal length, adjust and focus very carefuly and make the photo with your Polaroid 405 holder.
Don't move the camera after the shot otherwise the result may be worthless!!!
Take the photo and read the measures where the image frame ends -
e.g. the left side ends at the mark 11.5 inch on the paper stripe, mark 11.5 with a big colored mark on the stripe,
do so with the other sides.
Go back to the camera look on the viewing glass and you can see exactly
the edges of the image size - it's where you can see the four colored marks.
You can make some little marks with a permanent marker on the ground glass.
As you have now the exact measure of the image area of the instant film
you can now make your mask out of cardboard or opaque plastic sheet.