Just think about what your opponent wants at any time and how you can punish them for trying to get it. Be conscious of your own actions so that you're ready to counter the counter to what they expect in advanced. For general gameplay before you even look into what your opponent likes to do, you want to understand the value of stage positioning.
If you're going in with a read, go big or go home (Don't try to space out say, a stomp you intent to hit with, go in to make sure you'll hit with it; aim directly towards their expected position, or if you're just zoning, be multiple character lengths away). Either maximize reward on hit with a hard call out, or do advantageous positioning and soft reads that won't get you punished too hard if they miss (i.e. cover roll to center, jumping past, or jumping out of shield forward with a combo starter or a move with high knockback and not too low lag).
Cover yourself for when you might miss haymakers (like say, ASDIing down when throwing out stronger ground moves ready to Amsah tech, or SDI to the side to prevent a combo starting if you're hit).
You don't have to worry about lag or how safe the move you're throwing out as much when you're calling a change in position from where your opponent is with where you attack. To pressure their current position, you want to have good frame advantage on shield, whiff, or whatever you're doing and end up in a postion harder for them to cover.
You need a strong understanding of all of the "RPS" elements of the game: Moves with normal upwards and sideways knockback are good vs aerial opponents, downward vs grounded. Grab beats CC, shield, loses to dodges and disjoints, dodges beat grab, lose to active frames and waiting; etc etc. You figure out what the opponent would try to do to defend, you give them the option that beats it. All the while, put your opponent in bad situations where they can't retaliate as much as possible to see what they do to get out, and know how to cover it. Either keep them in the air if they go with upward combos or juggles, put them into knockdown on the ground over and over, or just push them offstage and closer to it while maintaining better position (or continually pressuring when they don't have time to fight back).
Remember that attacks aren't purely offense just as much as shields (or even rolls) aren't purely defense. You beat the opponent coming in by beating them to the punch (i.e. you run in with your own hitbox or just say, put out a bair when they're running in to overshoot), or by picking the right evasive option. You beat someone trying to cut you off coming in by not coming in as fast, or using your "defensive" options for offense. Likewise, you have to exploit the limitations of certain options people fall on (going airborne locking in movement to a degree, shield limiting range for subsequent moves, there only being so much stage to fall back on, the opponent needing space to convert dash to run to run cancel, stuff like that) while being aware of how those limitations could lead to you being exploited.
There's a lot of ways to put out moves that will cover several options at once (like say, covering 3/4 tech options with raptor boost, all the ways to cover rolls and jumps out of shield at the same with some larger or more active hitboxes), but it may serve you just as well to cover their best option (or the one you think their most likely to take, because say, it's the way out of what might cover all their other options at the time) with your stronger options. Pre-emptively putting a smash attack a la Falco's fsmash to cover that option when it's putting you away from all of their other options. Not unbeatable, but less likely to be punished, and if punished, likely less.
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The short is, be aware of the game's RPS nature and how to abuse it for the best way to understand their common options and deal with general options. If you pick the options that beat theirs and you put them into bad positions, they might start questioning their gameplan. And if you just put them into exploitable positions anyway constantly, you force them to make the decisions where you want to get the read on them. What they do out of disadvantage may inform you about what they'll do in other situations potentially less disadvantageous, but you have to understand why they're doing those options for that to work (and potentially just watching what they do next might tell you). Moreover, putting them in the bad positions is putting you closer to winning regardless of whether or not you're successfully reading their ways out anyway. Mango himself actually says most of what he does is just reaction, he's just very familiar with the game and all its situations and puts himself in good positions.
