Somewhere above Main, yes.
I'm not sure this qualifies as a tutorial, but I'll try to explain it in more depth.
make_command_list in Window_TitleCommand is the method which shows all your menu commands. Each command is added via a call to "add_command", which takes up to four arguments: the name of the command, its associated symbol, a flag determining whether it's enabled, and arbitrary extended data (which as far as I can tell is only used to record the skill type ID of skill commands in battle). For example, the default menu:
add_command(Vocab::new_game, :new_game)
add_command(Vocab::continue, :continue, continue_enabled)
add_command(Vocab::shutdown, :shutdown)
This shows that the window will have three commands: the first will be called whatever string is set by Vocab::new_game, which is "New Game". Its associated symbol will be :new_game. Second will be "Continue" with the symbol :continue. Whether it's enabled or not will be determined by the flag continue_enabled. Finally is the shut down command, which will be called "Shut Down" and its associated symbol will be :shutdown
I actually missed a step as the associated symbol is only the first part of incorporating custom commands. Now that you have your command list:
class Window_TitleCommand < Window_Command
def make_command_list
add_command("Story Mode" :story_mode)
add_command("Campaign Mode", :campaign_mode)
add_command("Options",

ptions)
add_command(Vocab::shutdown, :shutdown)
end
end
you set the handler methods for these symbols in Scene_Title:
class Scene_Title < Scene_Base
def create_command_window
@command_window.set_handler

story_mode, method

command_story_mode))
@command_window.set_handler

campaign_mode, method

command_campaign_mode))
@command_window.set_handler

options, method

command_options))
@command_window.set_handler

shutdown, method

command_shutdown))
end
end
set_handler is a method available to any selectable window and determines a method which will be called if the player chooses the command with the given associated symbol. For instance, as you've associated the symbol :story_mode with your "Story Mode" command, you can now call set_handler to have the command with the :story_mode symbol call the method command_story_mode.
Now you need to create methods for the commands to actually perform the things they're intended for. If you look at command_new_game for example, you'll see what happens when a player selects "New Game" in the default title menu.
def command_new_game
DataManager.setup_new_game
close_command_window
fadeout_all
$game_map.autoplay
SceneManager.goto(Scene_Map)
end
You'll have to make your own methods for story mode, campaign mode, and options.