I know how to create and use commands but how do I register them for singleplayer worlds?

What do you mean? There is an integrated server in the singleplayer world too, any command registered will work in singleplayer just as well as in multiplayer. Please elaborate on your question. Do you need to create client-side only commands or do you wish to know how to register commands in general?

I used the FMLServerStartingEvent to register the commands like this:
 

private void onServerStarting(final FMLServerStartingEvent e)
	{
		e.getCommandDispatcher().register(LiteralArgumentBuilder.<CommandSource>literal("celriff").then(GamemodeCreativeShortcutCommand.register()).then(GamemodeSurvivalShortcutCommand.register()).then(GamemodeSpectatorShortcutCommand.register()));
	}

 

but I am not able to use these commands in a single player world.

Okay, So I see a few errors with my code and fixed them. Firstly I wasn't using a SubscribeEvent annotation and Secondly, I was trying to register all of these commands as sub-commands to "celriff".