The United Nations Volunteers (UNV) regularly publishes selected volunteer opportunities. You must first create your profile in the UNV Global Talent Pool via a Unified Volunteering Platform to apply. Some assignments are not publicized because they are filled by candidates whose profiles are directly matched from the Global Talent Pool. Some sponsored assignments may have additional eligibility criteria, which you will find in the PDF file linked to the special call.
For more information, visit the official webpage of the United Nations Volunteers.