Step 1: Get the Camera
I chose the agfa clack, because it is robust, cheap and easy to modify. You can get this camera for a few bucks in online auctions.
If the housing is o.k., don't worry about the function. There is nearly nothing that can get broken.
Step 2: Get a Sheet of Aluminium
This is a very thin sheet of aluminium. You can get that from a soda-can, a tea candle or something similar.
Step 3: Make a Hole
The perfect hole is done in multiple steps. Do not just punch through, but make a small dent and polish it away. Dent to the other side and polish again. Do it as long as it takes to make a small hole.
Use this calculator for the best hole-size:
I chose 0.3mm for the smalles hole, resulting in a aperture of 256.
I checked the size of the hole with a lead of a mechanical pencil, which is 0.3mm.
By the way: The makro-pictures were taken with my smartphone where I applied a laser-lens from an old CD-ROM-drive to the camera.
Step 4: Make Another Hole
I wanted to have aperture 64 and 16.
Therefore I needed one more pin-hole of 1.25mm and used the existing hole that was already there.
An aperture of 16 doesn't make sense if you want to remove the lens.
I also checked these with leads of mechanical pencils or other stuff I found around in the house. 1.25mm is aproximately the size of a rather thick pin.
Step 5: Modify the Camera
Then I chose to black-out the back side to reduce reflections. Reflections can also be a nice feature, so try it without and if you don't like it, black-out afterwards.
You can use a candle to produce soot. But try it on some crap material first, because if you do it wrong you might destroy the aperture selector.
Step 6: Reinstall the Lens for Extra Sharpness
Step 7: Try Out the Modified Camera!
I have also taken account of the Schwarzschild-effect.
If you are working with very long exposures of some seconds and longer, the film doesn't react as calculated and you need to double the amount of time to give it the right exposure.
If you used the same appertures as I did, you can use the list below.
Take a lightmeter and measure at f8, then use the list to find the right exposure for your aperture:
The columns with the "+" include the Schwarzschild-correcture.