So I've always wanted to build a mechanical iris, and this 3D design contest sounds like the perfect way to work the bugs out.
We'll call this step 0 (maybe? it is my first instructable...) - Research! You need to figure out what you want to make and if its ever been made before. What problems they have had, or success stories. So hit up google images and search for mechanical iris.
the first image is what i actually ended up with!
Disclaimer: I know the mechanical iris has been instructabled to death, but this is focusing on the CAD design of it, not the mechanism itself.
Step 1: Draw It!
A 3D program is not a sketch book, make sure you have a plan before getting into it. I generally use AutoCAD to do the first drawings. I make a 2D drawing that gives the basics of the mechanism. Each component of the mechanism gets its own layer so I can turn the others off and get a clear picture of what I'm working on.