What I did...
For my PotatOS I decided I wanted her to be as accurate as possible. To do this, rather than getting a potato and molding it or making a generic potato shape, I used a program called 3D Ripper DX
which you can download for free to do so.
Running Portal 2 through the 3D Ripper means that when you take a screen shot, rather than just capturing an image, it captures the entire level, including the textures and saves them in a file format that is openable by 3DS Max, which you can get for free if you're a student, or you can use other free graphic software.
- To do this, open 3d Ripper DX, set where you want the files to save, what button you want to use as your capture button and launch the Portal2.exe file through it. This will launch your Portal 2 game with 3D Ripper DX running in the background. My PotatOS was taken from Chapter 6: The Fall. This scene is better because you only have the pipes and portal gun to clear out of your way and in game when she's attached to your gun she has no backing as I found out the first time I pulled out a scene. .To start Chapter 6 which thankfully the fall through the pipes is at the very begining of go to Main Menu > New Game > The Fall. The level will load with you falling alongside PotatOS so while she's talking away. Press your chosen capture button to capture the scene. Dont panic if the game freezes for a second, its just making sure it captures all the data from that one point. After it captures the scene you can exit the game.
- Open up 3ds Max (or whatever graphics software you are using) go to File > Import and find where you saved your files, open the Frames folder and import the .3dr file you captured. The textures wont load into the file but you can find them in the textures folder and rearrange them if you like, I just never found the need to plus I find it easier to see detail it it's one solid colour. You then need to clear out everyone else in the level so you have a clear shot of PotatOS
- From there you can render out various angles or like I said before take screen shots. At this point, before I went to tidy up my images, I deleted all the excess parts around the potato so all i was left with was the potato, and exported it as an .obj file (File > Export > chose what to export as and where). I then opened the .obj file in Pepakura Designer and printed out a test run of the potatOS part of the potato. This was only so i could guage size and then see if there were any bits I hadn't noticed.
- The next step with your screenshots or renders is to take them into Gimp, Photoshop, Illustration, or whatever graphics and illustration software you're using, create a new layer and trace over them with the pen tool so you're left with an outline of PotatOS. You'll need this for templates in the next section so it's good to take your time and do it right.
Now you have an reference images to work from it's time to get making! Keep the 3D model and the original shots of renders handy so you can check them from time to time, it's always nice to have then near by because sometimes a flat image doesn't convey a lump or bump so keep checking back to make sure you're going in the right direction!
Or you could..
If you wanted you could stop at the pepakura point and make your potato like mini Halo armour. I will warn you that because of the eyes in the potato some points can be a little tricky. Also if you wanted to do you ENTIRE
PotatOS it'd be a little trickier as the way the model for the game was made, all the extra bits in the model and stuck into the main Potato Body, so you'd need to cut into your pepakura potato when you're done to attach them, or trim them down.
Of course if you don't want to be messing around with those programs or your not that way inclined do not fret! Using images found online, or screenshots taken from the game there's no reason you can't make PotatOS by eye, it may be a little trickier, but it's a nice alternative. If you chose this method then you'll just need to use your own references for different parts when I'm using mine.
[EDIT: It was brought to my attention that 3D Ripper DX has some problems talking with Windows 7. This seems to effect only certain games, and Portal 2 seems to work ok, but if you want to use the 3D Ripper DX program for a different game using the same methods check out this chart that @Blackjosh on the tested.com forums passed onto me: http://www.lagspike.com/tools/3d_ripper_dx_compatibility/chart.html ]
Onward to Step 2!