To create a new job description, click on the Jobs option in the third block of the navigation sidebar.
From the list of jobs on this page, click on the one that needs a job description. If there is no job-description text already there for this job, you will see a large green "Create new job description" button. Click this and you will be taken to a page with a form to add the new job description.
Give your job description a title and type (or paste) the text into the text box. You can apply formatting to the text so that your job description is clear.
When you're done, click the 'Create job description' button at the bottom of the page and your job description will be saved.