First a disclaimer: I've only produced a few OoBs and none for at least month so I might have missed something. Also I realise that JMM will doubtless have this sort of thing on his redevelopment list and I'm surely way behind the game. Nevertheless....
Currently the process is
New army -> French/allied -> nation -> CinC -> new corps -> nation ->C.O. -> new unit -> nation ->attach to... -> type of unit -> regiment -> division -> brigade.
So, one passes from corps straight to units then back up to Divisions and down to Brigades
It seems clearer in my head to go
New army -> French/allied -> nation -> CinC -> new corps -> nation ->C.O. -> Division #1, Bde A, Bde B, Bde C -> Divison #2, etc.. -> then select a brigade and add new unit -> nation ->type of unit -> regiment.
Hence you build downwards. There's a problem with that?
