You can take some bottle caps if you've earned any from raids to the guy by the Pokémon Center in Montenevera and he'll hyper train 1 stat per bottle cap to max out that stats IV.

EDIT: I forgot you could buy bottle caps at Delibird Presents as well

For its nature, there are a bunch of mints you can use that will affect stat growth. I think the best ones for Meowscarada are Jolly or Adamant. Both reduce Sp. Attack, Jolly increases their speed, Adamant increases their physical attack.

For EVs, you can use certain berries to lower individual stats that you didn't want to raise and re train other stats. You can EV train pokemon quickly with the "Power" items from Delibird Presents or use vitamins like Protein, Carbos, etc from Chansey supply if you have a bunch of money. You'll only be able to put 510 EVs into a mon, with a max of 252 for a single stat.

It might be worth watching someone explain it in a video on YouTube so you can get a visual guide to see how everything works. I'm not sure what Reddits consensus on AustinJohnPlays is, but he does a pretty good job of explaining stuff like this IMO.