1. Obtain the Regis: You need to capture the three Regis: Regirock, Regice, and Registeel, first. You can find them in the following locations:
* Regirock: Found in the Snowpoint Temple, located in the eastern part of Snowpoint City.
* Regice: Found on the Route 213, south of the Snowpoint City. You'll need to battle it after solving a puzzle.
* Registeel: Found on Route 217, west of the Snowpoint City. You'll need to solve a puzzle to get to it.
2. Go to the Lake Acuity: Once you've captured all three Regis, head to the Lake Acuity. It's located on Route 216, north of the Snowpoint Temple.
3. Use the Regis to melt the ice: Bring your three Regis to the center of the lake. They will automatically activate, and melt the ice to reveal Regigigas!
4. Battle and Catch Regigigas: After the ice melts, you'll have to battle Regigigas. It's a very strong Pokemon, so prepare yourself! Once you defeat it, you can attempt to catch it.
Important Note: You cannot catch Regigigas until after you have defeated the Elite Four and become the champion.