You need roughly (or better I used roughly):

an old camera (e.g. a box-camera since they are easy to get and easy to work with - tipp: 6x9 cameras are perfect for the Raspberry)

Raspberry Pi Camera Module

2 LEDs (again the Nichia)

resistor (56 Ohm)

reflectors for the LEDs (18mm had fit perfectly)

a professional mounting for the camera module (or two clothespins)

wires

diffuser foil (really optional)

some extra LEDs and resistors for the countdown

I guess you get what is optional and what is needed for your project on your own.

Prepare:

Get rid of everything unneeded inside the camera - shutter, pressure plate, that thing which holds the film, mirrors. Also now it's a good time to read the next step where it comes to drilling holes in the back plate of the camera. This way the work on the body will be finished before you start with the inner parts.

The actual work:

On the front plate of the camera you glue and solder everything together. I guess pictures are better then words. The lenses of the former viewfinder are covered with the diffuser foil on which you place the reflectors for the LEDs. You place the LEDs in those of cause. Like for the flash both LEDs are getting one resistor in a row dividing the 9V.

Now to the camera module. You see the improvised fix for that. Maybe you can get something better, but it works...

The orange and green LEDs around the glas can be used for the countdown if there is no monitor attached to the camera. Actually one LED would be enough but I use two because I wanted to have different blinking patterns which will be in sync when the picture is taken - a little experiment on how such a countdown could be improved.