Instructions
Purchase several potions from a local PokeMart. Max Potions are always preferred, as they completely restore the HP of your Pokemon, but any potions will suffice. You will not be able to backtrack once you enter the dungeon that contains Deoxys, so you will want to have plenty of potions to keep your Pokemon alive.
Visit the optional dungeon Shining Hill. After completion of the game, an earthquake occurs, which causes the previously blocked dungeon to be accessible.
Travel to the dungeons seventeenth floor. The exit to each floor is usually always located on the far-right side of the dungeon and may contain a handful of obstacles in your way. Many of the dungeon's floors are just simple mazes that you have to work through to get to the next floor. Every fifteen steps or so, a wild Pokemon will challenge you to a battle. You won't be able to leave the dungeon and instantly regain your position, so you will want to use your potions wisely as you proceed to the dungeon's seventeenth floor.
Converse with Deoxys, who is at the back of the room. If you have the Sacred Slab in your inventory, you will be able to recruit the legendary Pokemon to join your party of Pokemon.