Progress through the main story until you unlock postgame content
Complete the main story
Unlock the Mewtwo-related postgame mission or event
Travel to the designated location for the Mewtwo encounter
Save your game before starting the encounter
Battle Mewtwo and weaken it
Use a Poké Ball to catch Mewtwo
If you fail, reload and try again
Check for any required update, event, or distribution if Mewtwo is not available in your game version