I have two tricks to train that, but most of it comes with experience, and careful studying of the game until you understand how each mechanic influences the decision making process. It's just like PPMD says : thinking comes before the game. Mostly.
The first of my tricks is to focus on one particular move of the opponent, and try to guess and punish when he/she will use it. It can be anything, a roll, a wavedash, a projectile, a getup option, a poke, a recovery route, anything, really. Just focus on that one thing until you know your opponent's patterns so well you can cause him to make that one move on command and profit.
This exercise teaches you to understand patterns in your opponent's mind, and abuse them. In a real match, if you've done this exercise enough, you will be able to see patterns without having to think about it.
The second is to focus on one of your own moves instead. Give yourself objectives such as "this stock, I'm going to kill with a fair", "this match, I'm going to land at least five dash attacks", "I'm not going to use any nair this stock", and stuff like that. Don't spam that one move you're supposed to land, though : make sure when you do use it, it works, by creating yourself the opportunity to use it.
This exercise lets you look at your own playstyle and get out of your own habits. In a real match, if you noticed that the opponent is more often punishing than taking risks, you will be able to adapt much quicker if you've done this exercise well enough.
Oh btw, whenever you train, you should ask yourself what are you training, and when is it going to be useful. You have to actually know what you're doing, basically. Nausicaa-sempai calls it being conscious of your own train of thoughts and this guy's a good teacher so listen to him.
With that said, if you don't understand notions like spacing, game phases, option covering, baiting and stuff, go look them up first. If you still wow at combos but don't wow at good dash dances yet, you've still got things to figure out before you can actually train your mind.
A couple more tricks that will get you out of pools but won't make you a good players because they only work on people who do not realize this :
-in a tech chase situation, people tend to roll to the nearest ledge if they are at low%, and towards the center of the stage if they are at kill %. Some people always roll to the center of the stage, others always roll away from you. Those are the three main patterns, and you should be able to recognize them as fast as possible.
-in a tech chase situation, if you jump and look like you'll land on your opponent, people tend to do a getup attack but you can double jump/use an air stall move to punish this
-it is easier to kill and to juggle someone who doesn't have his double jump anymore, so don't rush your juggles : threaten the opponent just enough that they will spend their double jump to gtfo, and then you can continue juggling, and kill a good 30% earlier than you would have otherwise.
-look at what the opponent does when he respawns and when you respawn. Most people always do the same thing every time.
-people who are at kill % tend to shield more, so they are easier to grab
-if you do not have to leave the center of the stage, don't feel forced to. It's a very advantageous position.
-there is no shame in camping. Those who think there is probably aren't good enough to counter it and you should teach them the hard way.
Oh and question everything you read. As far as you are concerned, I suck at this game.
