Instructions
Pick a partner Pokemon for your character. Your partner will be the Pokemon who will find and receive all Shiny Leaves. Once this Pokemon finds five Shiny Leaves, it will receive a leaf crown from Lyra or Ethan in New Bark Town.
Go to your partner Pokemon's profile and look up its nature. The Pokemon's nature will determine the routes where you might be able to find the Shiny Leaf. For example, Hasty Pokemon can only find the Shiny Leaf on Routes 11, 18, 26, 39 and 44. You can find a list of all the natures and corresponding routes on the Serebii website.
Fly or walk to one of the routes that corresponds with your Pokemon's nature. Walk in the long grass with your partner Pokemon. Every few minutes, talk to your Pokemon. If your Pokemon finds a Shiny Leaf, an explanation mark will pop up above its head, the Pokemon will dance and a green musical note will replace the explanation mark.