Correct spacing is the first skill you want to master with MK
Just to elaborate on this (wall of text alert):
Most people generally have an idea that good spacing is hitting with the tip of moves. So if you are hitting with the tip of your moves, that's good spacing.
But that isn't always true, when people are moving around and not hitting each other, there are actually ranges that you don't see immediately, that you're actually weaving in and out of.
Like, in a MK vs. Sheik match-up, you'd think that Sheik would have more range. She has long legs, she has that dash attack, dacus, etc... But actually, in that match-up, MK has the most range. Because if he tornadoes and moves forward.
So, both characters are moving and stuff, imagine you are playing as the Sheik. What you need to imagine is that he is almost always nadoing. So, the space between you and your opponent should be nado range.
What you want to be doing is always staying at the exact tip of your opponent's range and moving in and out. A lot of the time, this will bait them to come closer, and a lot of people think sub-consciously that once they get closer, they have to commit to an attack. So whenever you see the opponent enter that area of space I was talking about above, you should do something like jump, shield, dsmash, etc... Instead of trying to predict when he is gonna nado, think of it like "In case he nados...". It's just the safest option.
Also remember that what he is doing, affects this range. Like, say a Snake was in shield, he can't just DACUS straight out of shield. Same as when, like, MK is at full-jump height or whatever. That means that if you see MK jump, or Snake shield, you KNOW it is safe to get closer. Cause the range they have has changed. That is what pressure is, you don't actually need to hit someone to pressure them. Just by approaching when they are in that position, you've pressured them by limiting their options and forcing them to do something.
MK's nado probably wasn't a good example now that I think about it cause it's slow. But the basic idea is to stay just out of the range of the longest ranged attack they can perform immediately. So, Snake's DACUS, Fox's SH nair, etc... If you are in this position, you are just as safe as if you were at opposite ends of the stage. That also means that you shouldn't do things in that range that you wouldn't do at full stage. So, you wouldn't shield from full-stage away cause they can't hit you from that far out, so... Why should you shield when you are out of range of their farthest reaching attack?
And that is what real spacing really is, it's just staying out of the ranges (remember, different ranges for different situations) and seeing if they do something which lets you pressure them.