Salty SD is very easy to use and the thing you use to mod Smash 3DS, but I strongly recommend you dump your Smash 3DS or at least find a dump of it online, especially if you want to make your own music for the game.
In addition, I also recommend checking out Anemone 3DS and custom badge making programs, if you want free home menu themes and badges to spice up your bland default home menu without needing to pay for any of it.
You shouldn't rely on video tutorials to get things modded, for your information. Things are always moving in the hacking scene and videos get dated quick. Just stick to the text guides, but always pay attention to the dates stuff is published.
There are good mods on GameBanana such as Waluigi over Captain Falcon, Mr. Krabs over Wario, and I've made some funny voice mods and all but I haven't shared those. If you want one-slot voice mods, though, beware of mirror matches, they tend to mute the voices. There's a fix for the Wii U versions, but I haven't seen tools that fix the mtb that governs one-slot voice effects.