Just go to bed and sleep. Your soul will be separated from your body and to return, simply sleep again. You can only use your bed in your room to separate your soul from your body, but any bed will allow you to return to your body. When you are in soul form, the Grim Reaper will attack you, but you can avoid it by entering a room with a cross on the door. Be cautious of heavy breathing, as it signals that the Grim Reaper is nearby, and you better hide to a room.