Let me simplify it for you. First, you place a bounty on a the person. (Type '/bounty' for help.) Then, you kill that person in PVP and a head is generated in a few minutes. This is the basics...
Basically, do: /bounty place *name* 5000 (cheapest price) Then, do /bounty accept/confirm idk then wait 5 minutes Someone else then the person who placed the bounty has to do /bounty accept *target* and kill the person Then the person who killed the person gets the head.