For spacing you just retreat your attacks. It's the same idea. Fair, nair and bair are all good for spacing. Check out NintenJoe's most recent videos
here. In those videos it shows spacing nicely. That's how you should space. If your opponent likes to run off, try to use PK Thunder. Spacing is good, but it becomes almost useless against characters with projectile.
As for timing, you should follow one attack after another. In other words, try to chain attacks together, which would be comboing. So if you were in this situation where you used nair, you could follow it up with fair and then uair. You can also use fair > fair > dair. That one is a bit difficult to spike with, but it can work.
Ness doesn't really need to chase his opponents. If your opponent has no projectile, you don't need to follow them. You can attack at a distance with something such as PK Thunder. For someone like Yoshi, if he's just throwing his eggs at you, you should get close and do your best to avoid his eggs. Because if they're only being reliant on that, then getting close to them isn't going to do them any good.
If they block, grab them. If they spot dodge, find an attack that delays which will hit them when their spot dodge frames are gone. If they roll, just d-smash or dair them. Follow their patterns. Basically you're chasing them if they like to roll around. Anyway, you can use other stages to counterpick against Yoshi. So if he become reliant with his eggs, you can choose a stage with barriers.
Anyway, hopefully more people come in to help you out.
