A note to authorities
You may wish to write several notes to different friends and members from your family, but beyond this there might be more notes you may want to write. This depends on the formalities required by authorities in order to determine whether suicide was indeed the cause of death.
This could simplify formalities required to determine the cause of death and could minimize the effect of such formalities on your family.
A message to paramedics/police/coroner can affirm that it was suicide, describe the method chosen, explain how and why you decided to commit suicide, and provide information on how to contact relatives. It might include additional instructions for example, to deliver additional messages, to other people.