Instructions
Travel to a route that is connected to another route without any interruptions, such as cities, tunnels or buildings. Travel between these two routes, walking only in the portions covered with grass. If one or both of the routes does not have any grass, look for alternative routes.
Check your "PokéGear" to see if Latias is near you. Latias roams around from place to place, so it may take awhile for you to find him.
Spray yourself with "Repel." "Repel" will keep you from encountering regular Pokémon. You will still be able to encounter Latias, since he is a Legendary Pokémon.
Place a high-level (level 45 or higher) Pokémon at the front of your party; choose one that is fast and knows an attack such as "Hypnosis" or "Thunderwave." Latias will run away as soon as you encounter him, but you can paralyze him or put him to sleep to keep him from fleeing. The faster your Pokémon is, the greater the chance you will get to make the first attack.
Weaken Latias enough to capture him. You should avoid killing Latias, as you can only encounter him once. If you accidentally kill Latias, reload your game and start over.