Instructions
Obtain the National Pokedex from Professor Oak by visiting his lab in Pallet Town in the Kanto region.
Go to the Pokemon Fan Club in Vermillion City, go inside and then leave again. A non-player character called Steven Stone will approach and tell you that Latios is now roaming Kanto.
Open the Map Card on the PokeGear on the bottom screen of the DS and it will show the current location of all roaming Legendary Pokemon, including Latios.
Head to the area Latios is in quickly before it moves on and find it in the long grass. It is a large blue bird-like Pokemon that will be hard to miss.
Use Ice, Bug, Ghost, Dragon or Dark-type attacks to wear down Latios's health until it shows as a red bar and throw a Pokeball to catch it.