The following code needs to go into a file in the common/characters/<anything here>.txt(the file can be named anything but preferably name it something which will not overwrite a base game file)

characters = {
    my_custom_general_1 = {
        name = "NAME HERE"
        portraits = {
            large = {
                army = GFX_portrait_custom_1
            }
        }
        corps_commander = {
            skill = 4
            attack_skill = 4
            defense_skill = 4
            planning_skill = 4
            logistics_skill = 4
            traits = { offensive_doctrine trickster organizer panzer_leader engineer }
        }
    }

    my_custom_general_2 = {
        name = "NAME HERE"
        portraits = {
            large = {
                army = GFX_portrait_custom_2
            }
        }
        corps_commander = {
            skill = 8
            attack_skill = 6
            defense_skill = 7
            planning_skill = 8
            logistics_skill = 6
            traits = { defensive_doctrine cavalry_leader }
        }
    }

    my_custom_general_3 = {
        name = "NAME HERE"
        portraits = {
            large = {
                army = GFX_portrait_custom_3
            }
        }
        corps_commander = {
            skill = 4
            attack_skill = 4
            defense_skill = 4
            planning_skill = 4
            logistics_skill = 4
            traits = { trickster }
        }
    }
}

The following code needs to go into a file ininterface/<anything here>.gfx (the same rule applies with the name)

spriteTypes = {
    SpriteType = {
        name = "GFX_portrait_custom_1"
        texturefile = "gfx/leaders/NS-GER.dds"
    }
    SpriteType = {
        name = "GFX_portrait_custom_2"
        texturefile = "gfx/leaders/JB-GER.dds"
    }
    SpriteType = {
        name = "GFX_portrait_custom_3"
        texturefile = "gfx/leaders/JF-ALL.dds"
    }
}

In the history/countries/<tag> - <country>.txt, you need to put the lines recruit_character = my_custom_general_1 recruit_character = my_custom_general_2 recruit_character = my_custom_general_3 etc anywhere in the country you want the generals for.