I think the main thing is fully mastering all the movement options so that you can do them effortlessly consistently without mistakes, which does take considerable practice and training. After that you need to learn how each character counters/punishes every possible option. Then you figure out the optimal option in every situation(it's character dependant, but an example would be turnaround grabbing people behind you in turnaround grab range instead of back tilting, and the perfect autocancel timing for all the aerials). After you know what you are supposed to do in every situation, you learn to read and condition your opponent. Basically so that you know exactly what you are supposed to do in any given situation, and if you want to do something, you actually do it, instead of missing your inputs.
Personally I found the "Art of Smash" videos on youtube helpful. Not necessarily the character specific ones, but the general ones.
I recommend cloud (or lucina, if you're not pay to win) as an initial character, as he is the easiest. If you use patience and full utilization of movement options, you don't need any character specific knowledge to win. Your neutral will just be good, and that's enough to win.
After you master cloud you can try training with other characters if you want.
As far as training is concerned, I recommend just playing a match with infinite time against a mario Level 3 or less, and practice hitting it. You're good when you never miss, and never get hit.
When you get to character specifics, what you really need to know is a few things:
1.) What is my damage racking options at every percent?
2.) What are my kill options at every percent?
This is because basically you want to do number 1, followed by number 2. You want the highest damaging option, and then a kill move.
You will base your neutral on whatever options you have, but it's also kinda based on what options your opponent has. You need to know some other things:
3.) What is the best option against shield?
4.) What is the best option against aerials?
5.) What is my best option against projectiles?
6.) What is my best tech chase option?
7.) What is my best option against each of the options my opponent can use?
Let's say your opponent rolls away alot. You want to think to yourself, "what's my best option to punish my opponent rolling away?" If your opponent throws a particular move often, say, "How do I punish them for using this?"
Often times, with your options you will have one that does more damage or even kill, and one that is safer, in which case it's kinda to your discretion. But sometimes you have an option that is safer and more damaging, in which case, just use that.
That's just my 2 cents.