Things You'll Need
Instructions
Beat the Elite 4. You must finish the game by beating the Elite 4 in Pokemon Ruby before you can hope to find Latios.
Watch TV. In your trainer's room at home, there will be a report of an unusual Pokemon seen in the Hoenn region; this will be Latios.
Roam around Hoenn. After hearing the report on TV, you will have to roam around Hoenn a bit to finally catch up to Latios. You should have a Pokemon with the power to prevent Latios' retreat (like Wobbuffet) to have a chance at catching him.
Use a Master Ball. Only a Master Ball will have the power to contain Latios, so stock up before facing him.
Don't underestimate Latios. You may have beaten the Elite 4, but Latios is a fully-trained level 40 Pokemon. Plan to use potions or berries to heal your Pokemon and always try to prevent Latios from running away. Congratulations, you now have Latios.