Instructions
Complete your primary Pokemon journey by defeating the champions of the Pokemon League and the Kimono Girls. The three Kimono Girls, Zuki, Naoko and Sayo, are located in the Violet City, the Ilex Forest and the Ice Path, respectively.
Use your Pokemon's Fly ability to travel to Olivine City. Go to Professor Oak's home and speak with him. He will give you a National Pokedex, which will allow you to travel to the Johto region.
Travel to the Cianwood City area in the Johto region by using your Pokemon's Fly ability. Walk west until you reach the Embedded Tower. Groudon is located on the top floor of the Embedded Tower.
Battle Groudon and weaken it before attempting to catch it. Since Groudon is a Ground-type Pokemon, Water-, Ice- and Grass-type Pokemon are most effective in weakening the legendary Pokemon. After it has been weakened, use an Ultra Ball or Great Ball for a higher chance of catching Groudon.