Instructions
Open the inventory screen, select a Blue Eye Stone and press "Use." This will drop the stone on the ground where you are standing, and it will appear in the games of other players. This gives them the ability to summon you to fight alongside them as a Blue Phantom in their game. You can also summon other players to your game by pressing the "Use" button on Blue Eye Stones that they have left in your game world.
Use a White Eye Stone to cancel out the effects of the Blue Eye Stones. You can use a White Eye Stone to return to your own game if you are playing as a Blue Phantom, or to send a Blue Phantom out of your own game if you are the host. If you don't use a White Eye Stone, then the Blue Phantom will remain until you have completed the level it was summoned on.
Select and use a Black Eye Stone to invade another player's game as a Black Phantom. If you are able to manage to kill the other player you will return to your own game in human form rather than as a spirit. If you die in battle you will simply be sent back to your own world, but if you die accidentally or fall off a cliff then you will lose a Soul Level. Killing a Black Phantom rewards you with a Black Eye Stone.
Get a Red Eye Stone by killing the Maiden in Black at the end of the game. Using this stone places a Red Soul Sign on the ground which other players can use to summon you as a Black Phantom for a one-on-one battle. The loser of the fight loses a Soul Level, while the winner gets all of the Souls corresponding to that level.