Instructions
Find and use a Stone of Ephemeral Eyes. These can be purchased from traders, found as treasure and obtained by killing certain enemies, such as the giants in the Valley of Defilement.
Kill a world boss, either by yourself or as a blue phantom playing with another character. Each of the 16 worlds has its own boss. Bosses do not re-spawn like normal enemies after being killed, and you can only restore bosses by beginning the next game cycle by killing the False King.
Invade another player's game as a black phantom and kill the host player. You can invade another player's game by using the Black Eye Stone received from the Maiden in Black after killing Phalanx. You will be resurrected if you kill the host player, but you will lose a soul level if you die due to environmental damage, such as falling from ledges.
Join a player's game as a blue phantom and convince your host to use the Resurrection miracle. You can join a player's game by using the Blue Eye Stone received from the Maiden in Black after killing Phalanx, which will leave a blue marker that other players can activate to summon you to their game. The Resurrection miracle revives any blue phantoms near the caster and sends them back to their original world. Unfortunately, "Demon's Souls" does not allow for direct communication between players. Communicate with friends over forums and chat programs to coordinate player revivals.