Read this to get an understanding of how a remote trigger port works on a DSLR: http://www.doc-diy.net/photo/eos_wired_remote/
So essentially the focus+ground pins are triggered, and then the shutter+ground pins fire the camera.
The code should run on a small arduino (a nano would be awesome!).
Any additional circuity required should be easy to install (I recognize that a clock shield might be required. ... The device does NOT need a screen, or any way of changing the options on the physical unit. I would pre-program the arduino through a computer.
When bidding for the job, please send examples of similar projects that prove your skillset, and in your message please write "I understand the job description" - so that I know you have read the whole post.