Start off with the finished product, of course.
The camera was designed by a user named schlem who uploaded the camera to a website called Thingiverse, and is free to download under a creative commons license. What I'll do for you here is try to simplify the process of building one by describing what I did to get it functioning.
First, let me begin by giving credit where credit is due. Clem Alaniz Garza for giving us the opportunity, Joshua Garza and Jorge Garcia for their 3d printing expertise (I knew nothing about 3d printing other than it looked cool), Clarita Fajutag for helping coordinate the project, assemble the camera, and being a great wife while I was stressed in finishing this thing, and Marilyn Carren who made some amazing photographs using the final product.
When you download the files from Thingiverse, you may be slightly confused as there are a lot of files and no manual. You will find a very nice user manual for a pinhole camera called p6X6. It is not the same camera!
The picture I have provided shows you the essential parts you will need to build your first camera. Some of the .stl files are batched for convenience, but the downside is the printing of unnecessary parts which may be confusing and also extends the print time. The image provided has the parts labeled using the .jpg files provided in the download. Just find the corresponding .stl files and print.
Note: the build is done using the 50mm extension.
While you are waiting for your parts to print, go out and buy some superglue, a 2.5 allen wrench, three 0.5X8mm screws, and a 1/4-20 flanged nut. Also, you can begin making a 0.30mm pinhole. You can buy them online, or make one yourself. Here is a pretty good writeup for one.
Once you have your parts in hand, make sure to watch the video build a few times. I used my cellphone to record this, hence the less-than-optimal audio, but props to student Enrique Olguin for the fantastic explanation. We shot the video in one-take, with little preparation. He learned the actual names of all the parts before I did,
That's pretty much it. One major thing to notice is that we used a highly translucent PLA (print material) for this build. I would recommend you use a matte black PLA so that you don't have to do as we did and spend a few hours making sure no light gets through. Drop a bright LED inside your camera, close it up, and if you can see any light shining through in a dark room, it is not ready to use.