You need to prepare a job description, a statement about what you expect from the post-holder. But a job description also has formal status. Make a bulleted list about what you want the role to cover. Include at least one bullet which provides you flexibility eg “other duties as required to support colleagues”. At the top include the job title and their position in your organisation (to whom will they report?) and a short summary of the main purpose of the job.
Are there specific experience, skills or qualification which you need the new recruit to have? Include a list under the heading ‘Skills and Experience’.
Be specific. It is better to ask for familiarity with MS Excel and Outlook than the vague ‘computer literate’.