Follow the stops in order, from Twinleaf Town through the eight Gyms to the post-game. Each Pokémon is listed at the first point you can get it, including the Grand Underground as it opens up. Pokémon outside the Sinnoh Pokédex only appear after you get the National Pokédex. Tap a row to check it off. Evolves into shows later stages and how they evolve.
National Pokédex: see every Pokémon in the Sinnoh Pokédex, then show Professor Rowan. Shiny Charm: catch every non-Mythical Pokémon in the National Pokédex, then visit the game director at Hotel Grand Lake. In these games it only raises shiny odds for Eggs.
Ramanas Park
Ramanas Park opens on Route 221 after you complete the Sinnoh Pokédex. Buy slates with Mysterious Shards dug up in the Grand Underground, then place them in a room's pedestal to summon its legendary Pokémon (level 70). Rooms unlock in order, and a few are exclusive to one version. Checking a Pokémon here also checks it in the Pokédex.