I recently realised a good way to practice this (nairing and bairing landing)
Speeds up your game a lot when you have confidence in it.
Pick character of a various heights (Kirby I think is naturally the shortest) and in training mode set the CPU to 'Walk'. They just walk from one side of the stage to the other. Keep trying to land aerials on them either moving towards you or away. Helps spacing and timing.
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Yoshi MU can be a bit complicated, I personally feel eggs and down air are kinda the mental hurdles; punishing or approaching through eggs and successfully trapping/juggling Yoshi. If you're careful about how you jump it should really hamper how Yoshi ever gains a positional advantage over you, at his best he's hoping to trade with you favourably (as a heavier character) in grounded or aerial interactions and ZSS just has to focus on spacing and handling certain unique elements. Otherwise, zair beats everything yoshi does in the air horizontally. Be careful about Yoshi's nair out of shield as well, respect it, it means shielding after hitting shields a bit more often than trying to jab or up tilt.
Fox without customs is pretty easy, you outcamp him, you out damage him, you out combo him and you have a better recovery. You punish and trap the ****** and he's extreme combo fodder and dies very early. Dash attack is his most threatening tool here and will get consistent follow ups, expect it/play around it and you should be coming out on top (staple out of shield punishes like up-b, jab or up tilt are all pretty reliable except for his back air). You can get a bit trapped by up tilt but it's not that bad, be ready to avoid the up air or bair when you jump out of it, down-b can also help. In relation to his jab, the 1-2 repetition isn't guaranteed against ZSS most of the time, as she'll be able to jab Fox first every time, however Fox can still shield first and if you jab his shield you'll get punished, an awkward 50/50 but it's better than most characters (you'll probably get away with roll aways often).
Customs changed things... just the single one... the falco laser. Turns the match up into ZSS chasing Fox the entire time as he is going to do nothing but run away from you, you no longer outcamp him and getting in against it is a lot more complicated. The laser itself clashes (unlike falcos) so it's not too dire (our paralyzer is fine), but it still makes closing in on every action he makes (our mobilities are so similar, ZSS' aerial mobility will be noticeable clincher in a lot of situations) really tedious and you're likely getting dash attacked. However the destruction Fox receives at the foot of the heels is still pretty consistent. This is honestly THE match up where you should probably be taking the alternative blaster for ZSS, I haven't personally vested much time doing either (using the laser or using it against Fox), but I see the merit here more than elsewhere.