Okay, if you know how to make models in XSI, then you should be good to go in Sins. There is another step to convert the XSI file to a MESH file so that it can be used in the game, but it is well documented in the already mentioned forge tools. Somewhere in there you may have to set weapon points, hangar points, and ability points, but you can do that in notepad if necissary. Other than that you're new mesh should be ready for action.
