The FO3 modding community provides several tutorials on creating 3D models and bringing them into the G.E.C.K. editor. The most comprehensive of those appears to be the one originally made by Artisten as a great video tutorial, later turned into an excellent PDF document by Miax. There are also various adaptations and additions of the subject kindly provided by others.
Most of the tutorials center on loading a pre-made FO3 NIF model (like the infamous ashtray) into NifScope and replacing parts of this model with your own data. While this is a perfectly working solution (under most circumstances) it is also a rather complicated and tedious method. The Miax document, for example, is 112 pages strong and the NifScope process itself (including a rather complex method of converting the collision data) is explained in 47 steps on 28 pages! There is a lot that can go wrong along that path!
That is where this new and simple method comes in! The process described here is short and easy and seems to work perfectly for static objects and for movable items that can be pushed around, taken and dropped by the player. NifScope is not involved at all - you only need Blender and the NIF export plugin.
I am still experimenting with the method and there may be drawbacks, glitches or problems that I have not found yet. Therefore I would like to encourage you to test the method and provide any useful feedback you may have.
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The tutorial itself can be found here:
"http://www.fallout3nexus.com/downloads/download.php?id=66177"
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