1. Reach the summit of Mt. Coronet, located in the northwest corner of the Sinnoh region.
2. Enter Snowpoint Temple and head inside the main chamber, where you encountered the Legendary Pokemon "Palkia" if you're playing "Brilliant Diamond" or "Dialga" if you're playing "Shining Pearl."
3. Find the three locked doors inside the main chamber.
4. Collect the Regi items:
- To open the first door, you need the Regirock, which can be found on Route 206 by using Rock Smash on a boulder near the entrance of the cave on the western side.
- To open the second door, you need the Regice, which can be found on Route 216, where the Lake Acuity is located, by defogging the left ice tile.
- To open the third door, you need the Registeel, which can be found on Iron Island. To reach Iron Island, speak with the sailor in Canalave City and choose to ride the ferry to Iron Island. It can be found deep inside a cave on the island.
5. Place all three Regi items into their respective indentations in front of the three locked doors.
6. The doors will unlock, allowing you to enter the chamber where Regigigas is waiting for you.
7. Interact with the altar in the center of the chamber, which will summon Regigigas and initiate a battle.
It's important to note that Regigigas is a challenging opponent, so make sure you're well-prepared with strong Pokemon and healing items.