The person in possession of the DEATH NOTE is possessed by a god of death, its original owner, until they die.
S1 E15Unfinished Business
If a human uses the note, a god of death usually appears in front of him/her within 39 days after he/she uses the note.
Gods of death, the original owners of the DEATH NOTE, do not do, in principle, anything which will help or prevent the deaths in the note.
A god of death has no obligation to completely explain how to use the note or rules which will apply to the human who owns it.
A god of death can extend his life by putting human names on the note, but humans cannot.
A person can shorten his or her own life by using the note.
The human who becomes the owner of the DEATH NOTE can, in exchange of half of his/her remaining life, get the eyeballs of the god of death which will enable him/her to see a human's name and remaining lifetime when looking through them.
A god of death cannot be killed even if stabbed in his heart with a knife or shot in the head with a gun. However, there are ways to kill a god of death, which are not generally known to the gods of death.
The conditions for death will not be realized unless it is physically possible for that human or it is reasonable...
The specific scope of the condition for death is not known to the gods of death, either. So, you must examine and find out.
One page taken from the DEATH NOTE, or even a fragment of the page, contains the full effects of the note.
The instrument to write with can be anything, ((e.g. cosmetics, blood, etc)) as long as it can write directly on the note and remains as legible letters.
Even the original owners of the DEATH NOTE, gods of death, do not know much about the note.
You may also write the cause and/or details of death prior to filling in the name of the individual. Be sure to insert the name in front of the written cause of death. You have about 19 days (according to the human calendar) in order to fill in a name.
Even if you do not actually possess the DEATH NOTE, the effect will be the same if you can recognize the person and his/her name to place in the blank.
These quotes come from Wikiquote's Death Note pages and are available under the Creative Commons Attribution-ShareAlike License.