Launch the NTW3-7.0-setup.exe file, choose your language.
Select your NTW folder, by default it is "C:\Program Files (x86)\Steam\steamapps\common\Napoleon Total War" (warning: don't select the "data" folder like in v6)
Wait until all files are extracted.
Use the NTW3 launcher to start
Army selection
Because so many factions are now available, the weakest nations are now off screen if your resolution is not high enough. To select them you can find select them by using the pre-saved army setups: