Adding completely new units

[PlasmaKrab]

I don't think there's a through-and-through tutorial on creating units from scratch, since it isn't something you're supposed to do on a regular basis.

First thing to do is browse the MOBhack help so you know how units are modeled. Second, open SHPed and look at the help file for guidelines on creating icons (check the master icon list for unused icons like that of the Tu160).
Creating lbm images is relatively straightforward once you get the proper software, i.e. Paint Shop Pro v7.

Give a long hard study to all of that, and some trial and error should put you on the right way!