A Job Description sets out the tasks that are required to do the job and will typically include a job title, reporting lines, the main purpose of the job and tasks etc.
It may seem tedious but time spent before you go to the market can help you with the process itself. Spend time analysing what you need, get your Job Description and Person Specification right and it will help you focus on exactly what you are looking for when it comes to selection and interviewing.
