You just need to think about what your worst matchups, personally, are. So for a lot of Foxes, this means matchups like Marth on FD/Yoshi's, Peach on Yoshi's/FoD, Falco, or Sheik on DL64. Those matchups are just examples, and Fox arguably wins all of them, but they're ones I've seen a lot of people have trouble with. You need to find the ones you have trouble with, though.
So for example, I usually go to Fox as my default character. I have trouble against spacies who are faster than me though (which is most spacies around my level or better lol), and if I lose, I will take campy spacies to Yoshi's and go Peach, and aggressive spacies to FD and go Peach or Marth. Peach and Samus also give me trouble, so I will usually go Marth or Sheik against them instead.
It also helps to sort of have a go-to character for each stage, if some of them give you trouble. There is absolutely no matchup I dislike for Fox on DL64 or BF, so if those get counterpicked, I'm fairly comfortable going Fox always. The same goes for Yoshi's/FoD with Peach, and FD/Stadium with Marth. So if I'm not sure who to pick, I like to default to those characters.
Finally, some days a certain character just feels really on or off. I've had days where I've gone all Marth or all Peach just because it felt right, and days where I've completely ignored them both. This is one of the biggest advantages of having multiple characters imo, it can help mentally if you just don't feel it with one of them.
Hope this helps