How do you mod Xbox 360 fallout 3

You can, but to start with you would need a modded Xbox 360, and some extreme knowledge of the inner workings of the game itself. Even if you managed to mod the game, you would never be able to go online with it without being banned from XBL.

Now if you really don't care about being banned, keep in mind that the only kind of modding you be able to do with the 360 version of the game would be something like making yourself run faster, float, go through walls, alter the looks of a few items/people... You would never be able to take it as far as the computer modders go by recreating whole levels and missions and creating new weapons or NPCs. They can do some amazing stuff on the PC version. The 360? Not so much and too much hassle.