Instructions
Look under the Pokedex's "Area" tab for the roaming Pokemon that you wish to catch. The Pokemon will show its current location with a blinking red dot on the map.
Fly to the nearest city, and run into the grassy areas where the red dot was displayed to encounter the legendary Pokemon.
Throw a Master Ball at the roaming Pokemon to immediately capture it. If you don't have a Master Ball, throw Heavy Balls or Ultra Balls, as they are the most effective capturing devices to use on these creatures.
Place a Pokemon in the front of your team that has the ability Shadow Tag or Arena Trap to keep the roaming Pokemon from instantly running away. If you don't have a Pokemon with those abilities, substitute it with a speedy Pokemon that knows moves that prevent other Pokemon from escaping, such as Mean Look or Spider Web.
Disable your roaming target with attacks that induce the Sleep or Paralysis status effect. Disabling the Pokemon will increase the chance that the Poke Ball will work properly. Avoid using moves that cause Confusion, Burn or Poison, as these status effects may knock out the Pokemon before you can capture it.