To create a new job ad, 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 ad. If there is no job-ad text already there for this job, you will see a large green "Create new job ad" button. Click this and you will be taken to a page with a form to add the new advert.
Give your job ad a title and type (or paste) the text into the text box. You can apply formatting to the text so that your job ad is clear.
When you're done, click the 'Create job ad' button at the bottom of the page and your advert will be saved.