The character that you're using might change how you approach doing it.
For example, I find with Yoshi you need to wait a little bit longer than with the other characters with DJC before cancelling your double jump or you pretty much won't do anything because of how low Yoshi goes in his double jump animation. With Ness, go as fast as you want really, I've rarely gone too fast that my aerial doesn't get out. I don't know enough about Mewtwo and Peach to tell you about them.
In terms of methods, what I prefer is to just double tap X-->aerial, but I know that some people like to use a combination of X/Y and control stick, and another method you can use is to roll your thumb across X, Y, and A.
I don't know if some people prefer to use C-Stick when DJCing, I find it easier to use A because it lets me cancel faster since it's closer.
But overall, my recommendation is to go into training mode with whichever character you are gonna be using DJC with and get used to that character's specific DJC mechanics, use slow-mo if you like, and keep in mind that DJC has more uses than just being able to use aerials faster / closer to the ground than with a SH, you can also use it to surprise your opponent because of the awkward timing in which you begin to fall, and remember that you can of course FF out of it, which is really helpful in many situations.
Edit: Made this post before I saw you mention Mewtwo
Sorry, I don't know too much about Mewtwo, but I hope this helps!
