The short version is that approach using dashing shield until you push them into a corner. At that point they have to make some sort of decision. You read that decision, and you make a read and get crazy damage.
For instance:
* A very common loop you'll see with pikachu players is that they will full hop thunder jolt over and over followed by quick attack when you get close. The answer is to power shield walk or dashing power shield, and when you are close you either charge up smash, or shield and make a dash grab guess.
* lucas players often zair and pkfire, and then when you approach they tether grab. Again just power shield until you are in grab range, then roll towards him and reverse klaw/reverse standing grab. You can also spot dodge to grab/bowser bomb.
* one loop a shiek could use is needle and then fair when you approach. What I like to do when i'm close here is either roll behind and jab, or run backward and try to pivot grab the option immediately after the landing fair. For instance if they choose to do another short hop after that, the pivot grab read will scoop them up
Every player has there way of spacing, and of course they should have a ton of mix ups which can be frustrating. You just have to be even more confusing and unpredictable with your approaches. Rolls, short hop bairs, jabs, wavebounce firebreath. There's a ton of stuff you can do to make them feel like they can't keep you out. Just be random.