Instructions
Travel to the Ruins of Alph and enter the upper-left chamber of the area.
Place Ho-oh at the front of your party. This will solve the chamber's puzzle and allow you to delve further into the ruins.
Walk around until you encounter an "X" Unown. Finding one is a matter of chance, so it may take awhile for you to encounter the Pokemon.
Battle the Unown with a Bug-type or Dark-type Pokemon. Since it is a Psychic-type Pokemon, it is weak to both of these types of Pokemon. Do not use Fighting-type Pokemon against the Unown; its attacks are especially effective against them.
Weaken the Unown by attacking it. Deal it a status ailment --- such as Sleep, Paralysis or Frozen --- to increase your capture rate.
Throw a Poke Ball to capture the Unown.