Just work on stuff that you can master solo
(When I say CPUs you can sub in some of your friends, have them help train you by emulating certain scenarios/exchanges from high level matches you've seen.)
So....
Movement - this is a massive skill to be fluent in. Learn the stages like the back of your hand and be able to maneuver platforms and reach all points in the shortest possible time for the purpose of following up combos, landing quick KO moves, and for evasion/mindgames. If you were to spend just a solid year just mastering movement you'd be extremely hard to follow or predict by even high level players, movement is the most essential aspect of competitive play. Lay out a planned path for yourself where you navigate a stage like an obstacle course (add attacks and specials here and there) and record yourself and see where you can optimize. (i.e.
https://www.youtube.com/watch?v=LQNOD4cPTBo). You can even watch the top players tournament sets in slow mo and mimic exactly how they moved and attacked like shadow boxing.
Spacing and reaction - Play against CPUs like Mario/Doc, Ganon/Falcon, Peach, Zelda/Sheik and the Spacies. And just stand there and attack and evade only on reaction; nothing preemptive, and see how long you can go without taking damage Incorporate spot dodges, DD, and wavedashes. Try using only one move to 4 stock a CPU to learn the spacing of that certain move until it's muscle memory. When playing CPUs always keep in mind that they are not anything like humans so keep the mental awareness that you should ignore their habits, and you yourself should not expect those moves, recovery, or movement from a human. It's only practice concerning yourself, consider them on a casual level.
Mindgames - Develop different styles and attitudes of playing. Learn to cycle through them, the point is to never become predictable. Mang0 is a great example. From match to match and often within a match he will change his approach method many times, he overwhelms with options.
Become tricky with your movement, this can easily be practiced on CPUs when you dash at them then WD back for example they will go for an attack or grab, work on tricking the baiting the CPU into committing in neutral and on the ledge.
Punish game - Another very important aspect to the high level game you can practice against dumb CPUs (kinda). The most successful players take hits as far as they can, and they have the most damaging punishes laid out for every move in a MU. You can destroy a player just by waiting for them to throw out their laggiest moves and then punishing sufficiently. This is kind of the core of what wins and loses matches, but it's not often practiced or given thought by lower level players (they are often too worried about approaching and spamming tech skill). Look at frame data and learn the windows of end/start lag where you can capitalize.
Combos and MU knowledge- CPUs on lvl 4-5 and learn what combos work on diff. weights and fall speeds. Think about what attacks a competitive level Human would use against you and think about the danger zones of each character in relation to you. Reference the Master Hand DB or 20XX to figure out hitstun more in depth. It's wise to spend the most amount of time on the most played tournament characters.
Practice Powershielding, catching projectiles, nullifying projectiles (by hitting them, and learn what power of attack is needed for each)
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