Cosplayers and model creators will all give you fairly similar advice to starting a new project. It begins with a ton of hours spent on the Internet for research, and ends with paint, polish, and possibly some decals.
Psychobob has experience creating awesome 3D models of game characters and pieces. After the research, printing, sculpting and painting, this awesome model of Portal 2's Atlas was born.
His cyclops eye has even been wired to glow a soft blue. Creator Psychobob documented the construction of mini-Atlas over at his blog, which is an interesting look at what goes into building models like these.
3D Print: Atlas (Portal) Part1 [Psychobob Arts]