Both of my tutorials show you where to place the edited skin or decal files.
2:52-5:00
Skins
data\art\armies\<army>\vehicles\<vehicle>\skins\<ModID>_<season>\*.tga
3:05-4:22
Decals
data\art\armies\<army>\badges\<ModID>\*.tga
Error building rus.mod: Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index
Usually, this error happens the first time you try to build. Try rebuilding, but not cleaning. I think it just means that Mod Builder thinks your mod is empty.
Could you show how us you have your mod set up?