1. Catch all of the Pokémon that are native to the Sinnoh region. This includes Pokémon that you can find in the wild, as well as those that you can evolve from other Pokémon.
2. Trade for Pokémon that you can't find in Sinnoh. There are some Pokémon that can only be found in other regions, so you'll need to trade with other players if you want to complete your Sinnoh Pokédex.
3. Talk to NPCs: Talk to as many non-playable characters (NPCs) as you can in the game. Sometimes they will give you items, including TM, that can help you complete you pokedex with new registered Pokemon by evolving them. Be cautious when you go around talking to trainers as some of them will challenge you, depending on where in the storyline that they are. These NPCs are usually called "Pokemon trainers" while NPCs that can give useful items or advice in general, are termed "persons" in the list at the start menu.
Here are some specific tips for filling in your Sinnoh Pokédex:
- Use the Pokédex to keep track of which Pokémon you have seen and caught.
- Check the Habitat list in the Pokédex to find out where to find specific Pokémon.
- Use the GTS to trade for Pokémon that you can't find in Sinnoh.
- Look for special events and giveaways that offer rare Pokémon.