Install the Ad Manager extension and open Admin › Ads. An advert is either an image banner with a link or ad-network code such as Google AdSense.
Placements include the homepage zones, under the header, above, inside and below link lists, the top, side and bottom of link pages, the leaving page and above the footer. Each advert can have a start and end date and counts impressions and clicks.
Options: show an "Advertisement" label, hide ads for signed-in members, and never show ads on adult (18+) pages. Add your publisher line under SEO settings › Indexing and sitemap › ads.txt.