Only use your character's good moves, and, above all, know when and why to use them. This is the core of fundamentals.
What do you think the application of Link's d-tilt is? Is his f-tilt useful? Should I try to smash, or is it too slow and will the enemy recover from their stun before I can get it out? Then I should jab or tilt, again depending. Is he in the air and can I predict him coming down? Then I'll try to u-tilt him or f-smash where I think he's gonna land! Maybe, since Link's dash attack is crap, slow, and laggy, I should play defensively with item throwing or approaching or defending with nair.
This also means attacks in relation to space. Will the attack even reach?
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you always want to be at a range where your move will make you safe, if they block then due to shieldpush, but often times it's just not possible. Robin, when am I safe to charge?
The other major fundamental thing besides space and move management is predicting and mixing things up. If you become too predictable then you should be easily shut down. This doesn't mean you have to use your character's bad moves, just adjust the timing slightly, or rush in one time and don't in another, or grab instead. Am I charging Robin's laser? Then I should mix up how I'm gonna get out of their attack if they try to get close. When should I cancel it, and with what? Use a roll. Are they constantly hitting you into the air, and you can't get out? Try mixing up airdodges with drifting left or right or using your midair jump. Lure them into investment, then punish them when they do something risky.
Edit 3: these are fundamentals, but you'll notice that they require you to look at your opponent most times. This is true, in fact. You should never look at your own character. You must have such expertise that your peripherals, even your mind's peripherals, are enough for you to know your character. Your control should be sure, and steady, and the focus should be on how your opponents are doing and reacting.