The process of applying for a United Nations job starts on this website. Once you have explored the Job Openings and selected a role that interests you, and for which you have the required experience and qualifications, you can begin the process by selecting “Apply Now” on the Job Description page.
On this page, you will find step-by-step instructions for applying under Application Process, as well as guidelines for completing Application forms and preparing for an interview.
You will also find information about assessments for UN jobs under Information about Assessments, Examinations, and Tests.
Source: UN Careers.