As you have listed this question under arts, it sounds as if the point of the assignment is to demonstrate your understanding of potential careers in one of these fields, and to help you start researching them in order to develop a better understanding of your own career options upon graduation.
Your first step in researching this would be to decide what sort of posting this would involve, whether in a private practice of some sort, a hospital, working at a school as an art therapist, or teaching art therapy to future art therapists in a post-secondary educational environment.
Once you have determined the particular nature of the position, you need to consider appropriate credentials, such as undergraduate or graduate degrees, medical degrees, licenses, and Registered Expressive Arts Therapist or Registered Expressive Arts Consultant/Educators credentials.
Your next step would be looking at job search sites to determine common salary ranges for the specific job you have chosen. Once you have completed this research, you should create a posting that organizes crucial information in a clear and comprehensible fashion, e.g.:
- Opening for a [job title]
- Employer Name: [real or fictional]
- Employer contact information
- Hours: [full or part time, shift, overtime, travel, days]
- Salary range
- Job description [what tasks would job involve]
- Required qualifications [degrees, certification, experience]
- Preferred qualifications
- Non-discrimination [in some countries you are required to explicitly state your compliance with equal opportunity or similar legislation, including accommodations for disabilities]
Your main strategy for filling in this information would be to use a composite of 3-4 actual job postings for a job that is of particular interest to you.