This article takes you on a step by step journey to using GECK, the Garden of Eden Creation Kit, to build your own custom Vault in the PC video game Fallout 3. Just follow the steps outlined and you'll soon be Vaulting your way to a new level of entertainment, and inviting your friends to try.
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World Building 101

Fallout 3 world building for the PC is a relatively challenging process that is both extremely difficult but very engaging, satisfying, and fun to use. World building in the video game Fallout 3 is still relatively straight forward, though your first project will take about three hours to complete, depending on your computing background computers.
You will need a few programs in order to complete the job and make it possible for you to use Fallout 3's world building interface. You will need the Fallout 3 game in order to do this job and the GECK, Garden of Eden Creation Kit, which can be downloaded at geck.bethsoft.com, and then installed in your Fallout 3 directory. Don't worry if you happen to receive a "libvorbilfile.dll" error, this just means you have put the file in the wrong place and need to delete it and download it again.
Information and data on the GECK can be read and uploaded at the official GECK wiki at snipurl.com/9ohko and the Bethesda basic tutorials in text and video form at snipurl.com/9ohlp. The official mod forum for GECK can be found at snipurl.com/9ohmb.
Downloading the GECK
After you have downloaded the GECK at the link above, opening the existing Fallout 3 content is necessary, so you can start editing the content. In order to do this, head to File > Data and then double-click Fallout 3.esm on the list that appears. Now, select Okay to fill all the Fallout 3 locations. The next thing to do, before were able to start creating our new world, is to save your project so that you have a file to put all your stuff into. This is accomplished by selecting File > Save and then give it an unforgettable name.
A different reality
You can't actually change Fallout 3's main file, it won't allow this to happen. Instead, the GECK makes plug-ins that you can turn on or off using the handy launch screen that appears just prior to the game starting. Using this screen you can add content, play with the existing stuff, or completely change the areas that already exist without altering your original game. It's not a good idea to save with a mod activated and then later load with it turned off because you will receive a warning message that will tell you things might be a little off the wall. Instead create a new profile or at the very least a new character, and this way you should avoid destroying your work.
Making a vault
The safest and quickest way to create a new area is to copy one that is already made. The Vaults are the easiest to make, and lots of them already exist. Check out the Cell View, making sure you have chosen Interiors (external wasteland areas are handled differently, using height-maps for terrain) and choose one to use, just right-click on it and select Duplicate Cell. The new cell should show up beneath the old on, with COPY0000 as its name. Left-click it a few times and then choose a new ID, you can pretty well use anything you want too, but make it something you will easily remember. Now, select Edit, and open the Interior Data tab to give it a new title.
Clean the slate
Once the map is loaded all its components show up on the right of the Cell View panel. Clean things up using select all the objects in the list, and then click Delete. You will have a clean slate, but with stuff like the lighting settings left in place. Before adding things, select Snap To grid located on the top bar, and travel through the Object Window to Static > Dungeon > Vault. Now, create an Overseer's Office (VRmOverseerOffice01) by dragging it into the 3D window.
Turning the lights on
The screen window will be black when it first appears, but not to worry. Press A and the lights will magically turn on in the editor. You will need to move around the editor to get a better view (or to find the room if you can't presently see it) using the mouse wheel to zoom in and out and move the mouse while holding Shift to rotate the picture.