1st make sure to full screen. Then first bear you meet will give you instruction on how to play.
Basically when you kill an enemy you have to go to the Ghost world in less than 8 second after killing them. You have to kill them again in the ghost world. If you kill an enemy and don't kill it again in the ghost world he will respawn. To go to the ghost world you have to hold Right Mouse Button for 1 second.